ISO/IEC 10646:2003:
Information Technology – Universal Multiple-Octet Coded Character Set (UCS) plus Amendment 1:2005, Amendment 2:2006, Amendment
3:2008, and Amendment 4:2008, plus additional amendments and corrigenda, or successor
If
mis less than zero, return the String concatenation of the String
"-"and
ToString(-
m).
If
mis
+∞, return the String
"Infinity".
Otherwise, let
n,
k, and
sbe integers such that
k≥ 1, 10
k-1≤
s< 10
k, the Number value for
s× 10
n-
kis
m, and
kis as small as possible. Note that
kis the number of digits in the decimal representation of
s, that
sis not divisible by 10, and that the least significant digit of
sis not necessarily uniquely determined by these criteria.
If
k≤
n≤ 21, return the String consisting of the code units of the
kdigits of the decimal representation of
s(in order, with no leading zeroes), followed by
n-
koccurrences of the code unit 0x0030 (DIGIT ZERO).
If 0 <
n≤ 21, return the String consisting of the code units of the most significant
ndigits of the decimal representation of
s, followed by the code unit 0x002E (FULL STOP), followed by the code units of the remaining
k-
ndigits of the decimal representation of
s.
If -6 <
n≤ 0, return the String consisting of the code unit 0x0030 (DIGIT ZERO), followed by the code unit 0x002E (FULL
STOP), followed by -
noccurrences of the code unit 0x0030 (DIGIT ZERO), followed by the code units of the
kdigits of the decimal representation of
s.
Otherwise, if
k= 1, return the String consisting of the code unit of the single digit of
s, followed by code unit 0x0065 (LATIN SMALL LETTER E), followed by the code unit 0x002B (PLUS SIGN) or the
code unit 0x002D (HYPHEN-MINUS) according to whether
n-1 is positive or negative, followed by the code units of the decimal representation of the integer
abs(
n-1) (with no leading zeroes).
Return the String consisting of the code units of the most significant digit of the decimal representation of
s, followed by code unit 0x002E (FULL STOP), followed by the code units of the remaining
k-1 digits of the decimal representation of
s, followed by code unit 0x0065 (LATIN SMALL LETTER E), followed by code unit 0x002B (PLUS SIGN) or the code
unit 0x002D (HYPHEN-MINUS) according to whether
n-1 is positive or negative, followed by the code units of the decimal representation of the integer
abs(
n-1) (with no leading zeroes).
否则，让
n ，
k 和
s 是整数，使得
k ≥1,10
var> k -1 ≤
s ＆lt; 10
k ，
s的数值×10
n -
k 是
m ，
k 尽可能小。 如果
s 有多种可能，请选择
s 的值 x 10
n -
k 的值最接近
m 。 如果有
s 的两个这样的值，请选择一个是均匀的。 请注意，
k 是
s 的十进制表示中的位数，并且
s 不能被10整除。
注意 3
ECMAScript的实现者可能会发现David M. Gay为浮点数的二进制到十进制转换而编写的论文和代码很有用:
If
x and
y are exactly the same sequence of code units (same length and same code units at corresponding indices),
return
true; otherwise, return
false.
比较
x ＆lt;
y ，其中
x 和
y 是值，生成
true ，
false 或
undefined （表示至少有一个操作数是
NaN ）。除了
x 和
y ，算法将一个名为
LeftFirst 的布尔标志作为参数。该标志用于控制在
x 和
y 上执行具有潜在可见副作用的操作的顺序。这是必要的，因为ECMAScript指定从左到右的表达式的执行。
LeftFirst 的默认值为
true ，表示
x 参数对应于
y 参数的相应表达式。如果
LeftFirst 是
false ，则反之亦然，必须在
y 之前执行操作，然后再在
x 。这样的比较如下进行:
If
py is a prefix of
px, return
false. (A String value
p is a prefix of String value
q if
q can be the result of concatenating
p and some other String
r. Note that any String is a prefix of itself, because
r may be the empty String.)
If
px is a prefix of
py, return
true.
Let
k be the smallest nonnegative integer such that the code unit at index
k within
px is different from the code unit at index
k within
py. (There must be such a
k, for neither String is a prefix of the other.)
Let
m be the integer that is the code unit value at index
k within
px.
Let
n be the integer that is the code unit value at index
k within
py.
If
m <
n, return
true. Otherwise, return
false.
Else,
Let
nx be ?
ToNumber(
px). Because
px and
py are primitive values evaluation order is not important.
If
nx and
ny are the same Number value, return
false.
If
nx is
+0 and
ny is
-0, return
false.
If
nx is
-0 and
ny is
+0, return
false.
If
nx is
+∞, return
false.
If
ny is
+∞, return
true.
If
ny is
-∞, return
false.
If
nx is
-∞, return
true.
If the mathematical value of
nx is less than the mathematical value of
ny —note that these mathematical values are both finite and not both zero—return
true. Otherwise, return
false.
抽象操作DefinePropertyOrThrow用于调用对象的[[DefineOwnProperty]]内部方法，方法是在不能执行请求的属性更新时引发
TypeError 异常。 使用参数
O ，
P 和
desc 调用操作，其中
O 是对象，
P 是属性键，
desc 是
属性描述符(Property Descriptor)。 此抽象操作执行以下步骤:
抽象操作Invoke用于调用
ECMAScript语言值(ECMAScript language value)。 使用参数
V ，
P 和可选的
argumentsList 调用该操作，其中
V 用作查找属性和提供调用的
this值，
P 是属性键，
argumentsList 是传递的参数值的列表 的方法。 如果
argumentsList 不存在，则新的空
List用作其值。 此抽象操作执行以下步骤:
If
desc.[[Enumerable]] is
true, append
key to
names.
Order the elements of
names so they are in the same relative order as would be produced by the Iterator that would be returned if
the
EnumerateObjectProperties internal method was invoked with
O.
Let
envRec be the declarative
Environment Record for which the method was invoked.
Assert:
envRec does not already have a binding for
N.
在envRec中为N创建可变绑定，并记录它是未初始化的。 如果D为true，则记录新创建的绑定可能会被随后的删除绑定调用删除(Create a mutable binding in
envRec for
N and record that it is uninitialized. If
D is
true, record that the newly created binding may be deleted by a subsequent DeleteBinding call.)
具体的
环境记录Environment Record方法CreateImmutableBinding用于声明性环境记录创建一个 未初始化的名称
N 的新的不可变绑定。
环境记录中必须不存在
N。 如果布尔参数
S 具有值
true ，则新绑定被标记为严格绑定.
Let
envRec be the declarative
Environment Record for which the method was invoked.
Assert:
envRec does not already have a binding for
N.
Create an immutable binding in
envRec for
N and record that it is uninitialized. If
S is
true, record that the newly created binding is a strict binding.
具体的
环境记录Environment RecordSetMutableBinding方法用于声明性环境记录的尝试将名称为参数
N 的值的标识符的当前绑定的绑定值更改为参数
V 的值。 通常已经存在
N 的绑定，但在极少数情况下可能不存在。 如果绑定是不可变绑定，则如果
S 为
true ，则抛出
TypeError .
Let
envRec be the declarative
Environment Record for which the method was invoked.
For each property of the Global Object specified in clause
18, do
Let
name be the String value of the property name.
Let
desc be the fully populated data property descriptor for the property containing the specified attributes
for the property. For properties listed in
18.2,
18.3, or
18.4 the value of the [[Value]] attribute is the corresponding intrinsic object from
realmRec.
Let
callerScriptOrModule be
callerContext's ScriptOrModule.
Let
pending be PendingJob{ [[Job]]:
job, [[Arguments]]:
arguments, [[Realm]]:
callerRealm, [[ScriptOrModule]]:
callerScriptOrModule, [[HostDefined]]:
undefined }.
Perform any implementation or host environment defined processing of
pending. This may include modifying the [[HostDefined]] field or any other field of
pending.
Add
pending at the back of the Job Queue named by
queueName.
If the host requires use of an exotic object to serve as
realm's
global object, let
global be such an object created in an implementation defined manner. Otherwise, let
global be
undefined, indicating that an ordinary object should be created as the
global object.
If the host requires that the
this binding in
realm's global scope return an object other than the
global object, let
thisValue be such an object created in an implementation defined manner. Otherwise, let
thisValue be
undefined, indicating that
realm's global
this binding should be the
global object.
Create any implementation defined
global object properties on
globalObj.
In an implementation dependent manner, obtain the ECMAScript source texts (see clause
10) and any associated host-defined values for zero or more ECMAScript scripts and/or ECMAScript modules. For each such
sourceText and
hostDefined,
If
sourceText is the source code of a script, then
If
O is not
undefined, create an own data property named
P of object
O whose [[Value]], [[Writable]], [[Enumerable]] and [[Configurable]] attribute values are described by
Desc. If the value of an attribute field of
Desc is absent, the attribute of the newly created property is set to its default value.
If
O is not
undefined, create an own accessor property named
P of object
O whose [[Get]], [[Set]], [[Enumerable]] and [[Configurable]] attribute values are described by
Desc. If the value of an attribute field of
Desc is absent, the attribute of the newly created property is set to its default value.
Return
true.
Return
true, if every field in
Desc is absent.
Return
true, if every field in
Desc also occurs in
current and the value of every field in
Desc is the same value as the corresponding field in
current when compared using the
SameValue algorithm.
If the [[Configurable]] field of
current is
false, then
Return
false, if the [[Configurable]] field of
Desc is
true.
Return
false, if the [[Enumerable]] field of
Desc is present and the [[Enumerable]] fields of
current and
Desc are the Boolean negation of each other.
If
O is not
undefined, convert the property named
P of object
O from a data property to an accessor property. Preserve the existing values of the converted property's
[[Configurable]] and [[Enumerable]] attributes and set the rest of the property's attributes to their default
values.
Else,
If
O is not
undefined, convert the property named
P of object
O from an accessor property to a data property. Preserve the existing values of the converted property's
[[Configurable]] and [[Enumerable]] attributes and set the rest of the property's attributes to their default
values.
Assert:
functionKind is either
"normal",
"non-constructor" or
"generator".
If
functionKind is
"normal", let
needsConstruct be
true.
Else, let
needsConstruct be
false.
If
functionKind is
"non-constructor", let
functionKind be
"normal".
Let
F be a newly created ECMAScript function object with the internal slots listed in
Table 27. All of those internal slots are initialized to
undefined.
Set
F's essential internal methods to the default ordinary object definitions specified in
9.1.
Set
F's [[Call]] internal method to the definition specified in
9.2.1.
If
needsConstruct is
true, then
Set
F's [[Construct]] internal method to the definition specified in
9.2.2.
Set the [[ConstructorKind]] internal slot of
F to
"base".
Set the [[Strict]] internal slot of
F to
strict.
Set the [[FunctionKind]] internal slot of
F to
functionKind.
Set the [[Prototype]] internal slot of
F to
functionPrototype.
Set the [[Extensible]] internal slot of
F to
true.
Insert
d as the first element of
functionsToInitialize.
Let
argumentsObjectNeeded be
true.
If the value of the [[ThisMode]] internal slot of
func is
lexical, then
NOTE Arrow functions never have an arguments objects.
Let
argumentsObjectNeeded be
false.
Else if
"arguments" is an element of
parameterNames, then
Let
argumentsObjectNeeded be
false.
Else if
hasParameterExpressions is
false, then
If
"arguments" is an element of
functionNames or if
"arguments" is an element of
lexicalNames, then
Let
argumentsObjectNeeded be
false.
For each String
paramName in
parameterNames, do
Let
alreadyDeclared be
envRec.HasBinding(
paramName).
NOTE Early errors ensure that duplicate parameter names can only occur in non-strict functions that do not have parameter
default values or rest parameters.
NOTE mapped argument object is only provided for non-strict functions that don't have a rest parameter, any parameter
default value initializers, or any destructured parameters.
Let
iteratorRecord be
Record {[[Iterator]]:
CreateListIterator(
argumentsList), [[Done]]:
false}.
If
hasDuplicates is
true, then
Perform ? IteratorBindingInitialization for
formals with
iteratorRecord and
undefined as arguments.
Else,
Perform ? IteratorBindingInitialization for
formals with
iteratorRecord and
env as arguments.
If
hasParameterExpressions is
false, then
NOTE Only a single lexical environment is needed for the parameters and top-level vars.
Let
instantiatedVarNames be a copy of the
ListparameterNames.
For each
n in
varNames, do
If
n is not an element of
instantiatedVarNames, then
Append
n to
instantiatedVarNames.
Perform !
envRec.CreateMutableBinding(
n,
false).
Call
envRec.InitializeBinding(
n,
undefined).
Let
varEnv be
env.
Let
varEnvRec be
envRec.
Else,
NOTE A separate
Environment Record is needed to ensure that closures created by expressions in the formal parameter list do not have visibility of
declarations in the function body.
NOTE: Non-strict functions use a separate lexical
Environment Record for top-level lexical declarations so that a
direct eval can determine whether any var scoped declarations introduced by the eval code conflict with pre-existing top-level
lexically scoped declarations. This is not needed for strict functions because a strict
direct eval always places all declarations into a new
Environment Record.
Set the LexicalEnvironment of
calleeContext to
lexEnv.
Let
lexDeclarations be the LexicallyScopedDeclarations of
code.
For each element
d in
lexDeclarations do
NOTE A lexically declared name cannot be the same as a function/generator declaration, formal parameter, or a var
name. Lexically declared names are only instantiated here but not initialized.
Let
result be the
Completion Record that is the result of evaluating
F in an implementation defined manner that conforms to the specification of
F.
thisArgument is the
this value,
argumentsList provides the named parameters, and the NewTarget value is
undefined.
Let
result be the
Completion Record that is the result of evaluating
F in an implementation defined manner that conforms to the specification of
F. The
this value is uninitialized,
argumentsList provides the named parameters, and
newTarget provides the NewTarget value.
Assert:
steps is either a set of algorithm steps or other definition of a function's behaviour provided in this specification.
Let
func be a new built-in function object that when called performs the action described by
steps. The new function object has internal slots whose names are the elements of
internalSlotsList. The initial value of each of those internal slots is
undefined.
Let
target be the value of
F's [[BoundTargetFunction]] internal slot.
Let
boundThis be the value of
F's [[BoundThis]] internal slot.
Let
boundArgs be the value of
F's [[BoundArguments]] internal slot.
Let
args be a new list containing the same values as the list
boundArgs in the same order followed by the same values as the list
argumentsList in the same order.
Let
target be the value of
F's [[BoundTargetFunction]] internal slot.
Assert:
target has a [[Construct]] internal method.
Let
boundArgs be the value of
F's [[BoundArguments]] internal slot.
Let
args be a new list containing the same values as the list
boundArgs in the same order followed by the same values as the list
argumentsList in the same order.
If
SameValue(
F,
newTarget) is
true, let
newTarget be
target.
Assert:
oldLenDesc will never be
undefined or an accessor descriptor because Array objects are created with a length data property that
cannot be deleted or reconfigured.
Assert:
oldLenDesc will never be
undefined or an accessor descriptor because Array objects are created with a length data property that
cannot be deleted or reconfigured.
The abstract operation CreateMappedArgumentsObject is called with object
func, parsed grammar phrase
formals,
ListargumentsList, and
Environment Recordenv. The following steps are performed:
Assert:
formals does not contain a rest parameter, any binding patterns, or any initializers. It may contain duplicate
identifiers.
Let
len be the number of elements in
argumentsList.
Let
obj be a newly created arguments exotic object with a [[ParameterMap]] internal slot.
Set the [[GetOwnProperty]] internal method of
obj as specified in
9.4.4.1.
Set the [[DefineOwnProperty]] internal method of
obj as specified in
9.4.4.2.
Set the [[Get]] internal method of
obj as specified in
9.4.4.3.
Set the [[Set]] internal method of
obj as specified in
9.4.4.4.
Set the [[HasProperty]] internal method of
obj as specified in
9.4.4.5.
Set the [[Delete]] internal method of
obj as specified in
9.4.4.6.
Set the remainder of
obj's essential internal methods to the default ordinary object definitions specified in
9.1.
The abstract operation MakeArgGetter called with String
name and
Environment Recordenv creates a built-in function object that when executed returns the value bound for
name in
env. It performs the following steps:
An ArgGetter function is an anonymous built-in function with [[Name]] and [[Env]] internal slots. When an ArgGetter
function
f that expects no arguments is called it performs the following steps:
Let
name be the value of
f's [[Name]] internal slot.
Let
env be the value of
f's [[Env]] internal slot.
Return
env.GetBindingValue(
name,
false).
Note
ArgGetter functions are never directly accessible to ECMAScript code.
The abstract operation MakeArgSetter called with String
name and
Environment Recordenv creates a built-in function object that when executed sets the value bound for
name in
env. It performs the following steps:
An ArgSetter function is an anonymous built-in function with [[Name]] and [[Env]] internal slots. When an ArgSetter
function
f is called with argument
value it performs the following steps:
Let
name be the value of
f's [[Name]] internal slot.
Let
env be the value of
f's [[Env]] internal slot.
When the [[DefineOwnProperty]] internal method of an Integer Indexed exotic object
O is called with property key
P, and
Property DescriptorDesc, the following steps are taken:
When the [[Get]] internal method of an Integer Indexed exotic object
O is called with property key
P and
ECMAScript language valueReceiver, the following steps are taken:
When the [[Set]] internal method of an Integer Indexed exotic object
O is called with property key
P, value
V, and
ECMAScript language valueReceiver, the following steps are taken:
The abstract operation IntegerIndexedObjectCreate with arguments
prototype and
internalSlotsList is used to specify the creation of new Integer Indexed exotic objects. The argument
internalSlotsList is a
List of the names of additional internal slots that must be defined as part of the object. IntegerIndexedObjectCreate performs
the following steps:
Assert:
internalSlotsList contains the names [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]].
Let
A be a newly created object with an internal slot for each name in
internalSlotsList.
Set
A's essential internal methods to the default ordinary object definitions specified in
9.1.
Set the [[GetOwnProperty]] internal method of
A as specified in
9.4.5.1.
Set the [[HasProperty]] internal method of
A as specified in
9.4.5.2.
Set the [[DefineOwnProperty]] internal method of
A as specified in
9.4.5.3.
Set the [[Get]] internal method of
A as specified in
9.4.5.4.
Set the [[Set]] internal method of
A as specified in
9.4.5.5.
Set the [[OwnPropertyKeys]] internal method of
A as specified in
9.4.5.6.
Set the [[Prototype]] internal slot of
A to
prototype.
Set the [[Extensible]] internal slot of
A to
true.
When the [[DefineOwnProperty]] internal method of a module namespace exotic object
O is called with property key
P and
Property DescriptorDesc, the following steps are taken:
When the [[Get]] internal method of a module namespace exotic object
O is called with property key
P and
ECMAScript language valueReceiver, the following steps are taken:
When the [[Set]] internal method of a module namespace exotic object
O is called with property key
P, value
V, and
ECMAScript language valueReceiver, the following steps are taken:
Let
target be the value of the [[ProxyTarget]] internal slot of
O.
Let
trap be ?
GetMethod(
handler,
"isExtensible").
If
trap is
undefined, then
Return ?
target.[[IsExtensible]]().
Let
booleanTrapResult be
ToBoolean(?
Call(
trap,
handler, «
target »)).
Let
targetResult be ?
target.[[IsExtensible]]().
If
SameValue(
booleanTrapResult,
targetResult) is
false, throw a
TypeError exception.
Return
booleanTrapResult.
Note
[[IsExtensible]] for proxy objects enforces the following invariant:
The result of [[IsExtensible]] is a Boolean value.
[[IsExtensible]] applied to the proxy object must return the same value as [[IsExtensible]] applied to the proxy object's
target object with the same argument.
If
targetDesc is
undefined or
targetDesc.[[Configurable]] is
true, then
Throw a
TypeError exception.
Return
resultDesc.
Note
[[GetOwnProperty]] for proxy objects enforces the following invariants:
The result of [[GetOwnProperty]] must be either an Object or
undefined.
A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object
is not extensible.
A property cannot be reported as existent, if it does not exists as an own property of the target object and the target object
is not extensible.
A property cannot be reported as non-configurable, if it does not exists as an own property of the target object or if it
exists as a configurable own property of the target object.
When the [[DefineOwnProperty]] internal method of a Proxy exotic object
O is called with property key
P and
Property DescriptorDesc, the following steps are taken:
If
settingConfigFalse is
true and
targetDesc.[[Configurable]] is
true, throw a
TypeError exception.
Return
true.
Note
[[DefineOwnProperty]] for proxy objects enforces the following invariants:
The result of [[DefineOwnProperty]] is a Boolean value.
A property cannot be added, if the target object is not extensible.
A property cannot be non-configurable, unless there exists a corresponding non-configurable own property of the target object.
If a property has a corresponding target object property then applying the
Property Descriptor of the property to the target object using [[DefineOwnProperty]] will not throw an exception.
When the [[Get]] internal method of a Proxy exotic object
O is called with property key
P and
ECMAScript language valueReceiver, the following steps are taken:
Let
trapResult be ?
Call(
trap,
handler, «
target,
P,
Receiver »).
Let
targetDesc be ?
target.[[GetOwnProperty]](
P).
If
targetDesc is not
undefined, then
If
IsDataDescriptor(
targetDesc) is
true and
targetDesc.[[Configurable]] is
false and
targetDesc.[[Writable]] is
false, then
If
SameValue(
trapResult,
targetDesc.[[Value]]) is
false, throw a
TypeError exception.
If
IsAccessorDescriptor(
targetDesc) is
true and
targetDesc.[[Configurable]] is
false and
targetDesc.[[Get]] is
undefined, then
If
trapResult is not
undefined, throw a
TypeError exception.
Return
trapResult.
Note
[[Get]] for proxy objects enforces the following invariants:
The value reported for a property must be the same as the value of the corresponding target object property if the target
object property is a non-writable, non-configurable own data property.
The value reported for a property must be
undefined if the corresponding target object property is a non-configurable own accessor property that has
undefined as its [[Get]] attribute.
When the [[Set]] internal method of a Proxy exotic object
O is called with property key
P, value
V, and
ECMAScript language valueReceiver, the following steps are taken:
Let
booleanTrapResult be
ToBoolean(?
Call(
trap,
handler, «
target,
P,
V,
Receiver »)).
If
booleanTrapResult is
false, return
false.
Let
targetDesc be ?
target.[[GetOwnProperty]](
P).
If
targetDesc is not
undefined, then
If
IsDataDescriptor(
targetDesc) is
true and
targetDesc.[[Configurable]] is
false and
targetDesc.[[Writable]] is
false, then
If
SameValue(
V,
targetDesc.[[Value]]) is
false, throw a
TypeError exception.
If
IsAccessorDescriptor(
targetDesc) is
true and
targetDesc.[[Configurable]] is
false, then
If
targetDesc.[[Set]] is
undefined, throw a
TypeError exception.
Return
true.
Note
[[Set]] for proxy objects enforces the following invariants:
The result of [[Set]] is a Boolean value.
Cannot change the value of a property to be different from the value of the corresponding target object property if the corresponding
target object property is a non-writable, non-configurable own data property.
Cannot set the value of a property if the corresponding target object property is a non-configurable own accessor property
that has
undefined as its [[Set]] attribute.
The [[Call]] internal method of a Proxy exotic object
O is called with parameters
thisArgument and
argumentsList, a
List of ECMAScript language values. The following steps are taken:
Let
handler be the value of the [[ProxyHandler]] internal slot of
O.
A Proxy exotic object only has a [[Call]] internal method if the initial value of its [[ProxyTarget]] internal slot
is an object that has a [[Call]] internal method.
The [[Construct]] internal method of a Proxy exotic object
O is called with parameters
argumentsList which is a possibly empty
List of ECMAScript language values and
newTarget. The following steps are taken:
Let
handler be the value of the [[ProxyHandler]] internal slot of
O.
Let
newObj be ?
Call(
trap,
handler, «
target,
argArray,
newTarget »).
If
Type(
newObj) is not Object, throw a
TypeError exception.
Return
newObj.
Note 1
A Proxy exotic object only has a [[Construct]] internal method if the initial value of its [[ProxyTarget]] internal
slot is an object that has a [[Construct]] internal method.
Note 2
[[Construct]] for proxy objects enforces the following invariants:
全局代码(
Global code)是被视为ECMAScript脚本的源文本。 特定脚本的全局代码不包括被解析为FunctionDeclaration，FunctionExpression，GeneratorDeclaration，GeneratorExpression，MethodDefinition，ArrowFunction，ClassDeclaration或ClassExpression的一部分的任何源文本。
函数代码(
Function code)是解析为ECMAScript函数对象的[[ECMAScriptCode]]和[[FormalParameters]]内部插槽（见9.2）的值的源文本。 特定ECMAScript函数的函数代码不包括被解析为嵌套的FunctionDeclaration，FunctionExpression，GeneratorDeclaration，GeneratorExpression，MethodDefinition，ArrowFunction，ClassDeclaration或ClassExpression的函数代码的任何源文本。
The TRV of
UnicodeEscapeSequence::u{HexDigits} is the sequence consisting of code unit value 0x0075 followed by code unit value 0x007B followed by TRV of
HexDigits followed by code unit value 0x007D.
CreateDataProperty is used to ensure that own properties are defined for the array even if the standard built-in Array prototype object
has been modified in a manner that would preclude the creation of new own properties using [[Set]].
Return a
List whose first element is
siteObj, whose second elements is
firstSub, and whose subsequent elements are the elements of
restSub, in order.
restSub may contain no elements.
Append the
Record{[[Strings]]:
rawStrings, [[Array]]:
template} to
templateRegistry.
Return
template.
Note 1
The creation of a template object cannot result in an
abrupt completion.
Note 2
Each
TemplateLiteral in the program code of a
realm is associated with a unique template object that is used in the evaluation of tagged Templates (
12.2.9.5). The template objects are frozen and the same template object is used each time a specific tagged Template is evaluated.
Whether template objects are created lazily upon first evaluation of the
TemplateLiteral or eagerly prior to first evaluation is an implementation choice that is not observable to ECMAScript code.
Note 3
Future editions of this specification may define additional non-enumerable properties of template objects.
The abstract operation EvaluateCall takes as arguments a value
ref, a syntactic grammar production
arguments, and a Boolean argument
tailPosition. It performs the following steps:
The abstract operation EvaluateDirectCall takes as arguments a value
func, a value
thisValue, a syntactic grammar production
arguments, and a Boolean argument
tailPosition. It performs the following steps:
Let
argList be ? ArgumentListEvaluation(
arguments).
If
Type(
func) is not Object, throw a
TypeError exception.
If
IsCallable(
func) is
false, throw a
TypeError exception.
Assert: If
tailPosition is
true, the above call will not return here, but instead evaluation will continue as if the following return
has already occurred.
Return a value of type
Reference that is a Super
Reference whose base value is
bv, whose referenced name is
propertyKey, whose thisValue is
actualThis, and whose strict reference flag is
strict.
The sign of the result is positive if both operands have the same sign, negative if the operands have different signs.
Division of an infinity by an infinity results in
NaN.
Division of an infinity by a zero results in an infinity. The sign is determined by the rule already stated above.
Division of an infinity by a nonzero finite value results in a signed infinity. The sign is determined by the rule already
stated above.
Division of a finite value by an infinity results in zero. The sign is determined by the rule already stated above.
Division of a zero by a zero results in
NaN; division of zero by any other finite value results in zero, with the sign determined by the rule already
stated above.
Division of a nonzero finite value by a zero results in a signed infinity. The sign is determined by the rule already stated
above.
In the remaining cases, where neither an infinity, nor a zero, nor
NaN is involved, the quotient is computed and rounded to the nearest representable value using IEEE 754-2008
round to nearest, ties to even mode. If the magnitude is too large to represent, the operation overflows; the result
is then an infinity of appropriate sign. If the magnitude is too small to represent, the operation underflows and
the result is a zero of the appropriate sign. The ECMAScript language requires support of gradual underflow as defined
by IEEE 754-2008.
The sign of the result equals the sign of the dividend.
If the dividend is an infinity, or the divisor is a zero, or both, the result is
NaN.
If the dividend is finite and the divisor is an infinity, the result equals the dividend.
If the dividend is a zero and the divisor is nonzero and finite, the result is the same as the dividend.
In the remaining cases, where neither an infinity, nor a zero, nor
NaN is involved, the floating-point remainder r from a dividend n and a divisor d is defined by the mathematical
relation r = n - (d × q) where q is an integer that is negative only if n/d is negative and positive only if n/d is
positive, and whose magnitude is as large as possible without exceeding the magnitude of the true mathematical quotient
of n and d. r is computed and rounded to the nearest representable value using IEEE 754-2008 round to nearest, ties
to even mode.
Returns an implementation-dependent approximation of the result of raising
base to the power
exponent.
If
exponent is
NaN, the result is
NaN.
If
exponent is
+0, the result is 1, even if
base is
NaN.
If
exponent is
-0, the result is 1, even if
base is
NaN.
If
base is
NaN and
exponent is nonzero, the result is
NaN.
If
abs(
base) > 1 and
exponent is
+∞, the result is
+∞.
If
abs(
base) > 1 and
exponent is
-∞, the result is
+0.
If
abs(
base) is 1 and
exponent is
+∞, the result is
NaN.
If
abs(
base) is 1 and
exponent is
-∞, the result is
NaN.
If
abs(
base) < 1 and
exponent is
+∞, the result is
+0.
If
abs(
base) < 1 and
exponent is
-∞, the result is
+∞.
If
base is
+∞ and
exponent > 0, the result is
+∞.
If
base is
+∞ and
exponent < 0, the result is
+0.
If
base is
-∞ and
exponent > 0 and
exponent is an odd integer, the result is
-∞.
If
base is
-∞ and
exponent > 0 and
exponent is not an odd integer, the result is
+∞.
If
base is
-∞ and
exponent < 0 and
exponent is an odd integer, the result is
-0.
If
base is
-∞ and
exponent < 0 and
exponent is not an odd integer, the result is
+0.
If
base is
+0 and
exponent > 0, the result is
+0.
If
base is
+0 and
exponent < 0, the result is
+∞.
If
base is
-0 and
exponent > 0 and
exponent is an odd integer, the result is
-0.
If
base is
-0 and
exponent > 0 and
exponent is not an odd integer, the result is
+0.
If
base is
-0 and
exponent < 0 and
exponent is an odd integer, the result is
-∞.
If
base is
-0 and
exponent < 0 and
exponent is not an odd integer, the result is
+∞.
If
base < 0 and
base is finite and
exponent is finite and
exponent is not an integer, the result is
NaN.
Note
The result of
base**exponent when
base is
1 or
-1 and
exponent is
+Infinity or
-Infinity differs from IEEE 754-2008. The first edition of ECMAScript specified a result of
NaN for this operation, whereas later versions of IEEE 754-2008 specified
1. The historical ECMAScript behaviour is preserved for compatibility reasons.
Return the result of applying the addition operation to
lnum and
rnum. See the Note below
12.8.5.
Note 1
No hint is provided in the calls to
ToPrimitive in steps 5 and 6. All standard objects except Date objects handle the absence of a hint as if the hint Number were
given; Date objects handle the absence of a hint as if the hint String were given. Exotic objects may handle the absence
of a hint in some other manner.
Note 2
Step 7 differs from step 5 of the
Abstract Relational Comparison algorithm, by using the logical-or operation instead of the logical-and operation.
Return the result of applying the subtraction operation to
lnum and
rnum. See the note below
12.8.5.
12.8.5Applying the Additive Operators to Numbers
#
The
+ operator performs addition when applied to two operands of numeric type, producing the sum of the operands.
The
- operator performs subtraction, producing the difference of two numeric operands.
Addition is a commutative operation, but not always associative.
The result of an addition is determined using the rules of IEEE 754-2008 binary double-precision arithmetic:
If either operand is
NaN, the result is
NaN.
The sum of two infinities of opposite sign is
NaN.
The sum of two infinities of the same sign is the infinity of that sign.
The sum of an infinity and a finite value is equal to the infinite operand.
The sum of two negative zeroes is
-0. The sum of two positive zeroes, or of two zeroes of opposite sign, is
+0.
The sum of a zero and a nonzero finite value is equal to the nonzero operand.
The sum of two nonzero finite values of the same magnitude and opposite sign is
+0.
In the remaining cases, where neither an infinity, nor a zero, nor
NaN is involved, and the operands have the same sign or have different magnitudes, the sum is computed and
rounded to the nearest representable value using IEEE 754-2008 round to nearest, ties to even mode. If the magnitude
is too large to represent, the operation overflows and the result is then an infinity of appropriate sign. The ECMAScript
language requires support of gradual underflow as defined by IEEE 754-2008.
Note
The
- operator performs subtraction when applied to two operands of numeric type, producing the difference of its
operands; the left operand is the minuend and the right operand is the subtrahend. Given numeric operands
a and
b, it is always the case that
a-b produces the same result as
a+(-b).
Let
shiftCount be the result of masking out all but the least significant 5 bits of
rnum, that is, compute
rnum & 0x1F.
Return the result of performing a sign-extending right shift of
lnum by
shiftCount bits. The most significant bit is propagated. The result is a signed 32-bit integer.
Let
shiftCount be the result of masking out all but the least significant 5 bits of
rnum, that is, compute
rnum & 0x1F.
Return the result of performing a zero-filling right shift of
lnum by
shiftCount bits. Vacated bits are filled with zero. The result is an unsigned 32-bit integer.
The result of evaluating a relational operator is always of type Boolean, reflecting whether the relationship named by
the operator holds between its two operands.
12.10.4Runtime Semantics: InstanceofOperator(
O,
C)
#
The abstract operation InstanceofOperator(
O,
C) implements the generic algorithm for determining if an object
O inherits from the inheritance path defined by constructor
C. This abstract operation performs the following steps:
If
Type(
C) is not Object, throw a
TypeError exception.
Let
instOfHandler be ?
GetMethod(
C, @@hasInstance).
Steps 5 and 6 provide compatibility with previous editions of ECMAScript that did not use a @@hasInstance method to
define the
instanceof operator semantics. If a function object does not define or inherit @@hasInstance it uses the default
instanceof semantics.
The result of evaluating an equality operator is always of type Boolean, reflecting whether the relationship named by
the operator holds between its two operands.
If
r is
true, return
false. Otherwise, return
true.
Note 1
Given the above definition of equality:
String comparison can be forced by:
"" + a == "" + b.
Numeric comparison can be forced by:
+a == +b.
Boolean comparison can be forced by:
!a == !b.
Note 2
The equality operators maintain the following invariants:
A != B is equivalent to
!(A == B).
A == B is equivalent to
B == A, except in the order of evaluation of
A and
B.
Note 3
The equality operator is not always transitive. For example, there might be two distinct String objects, each representing
the same String value; each String object would be considered equal to the String value by the
== operator, but the two String objects would not be equal to each other. For example:
new String("a") == "a" and
"a" == new String("a") are both
true.
new String("a") == new String("a") is
false.
Note 4
Comparison of Strings uses a simple equality test on sequences of code unit values. There is no attempt to use the more
complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode
specification. Therefore Strings values that are canonically equal according to the Unicode standard could test as
unequal. In effect this algorithm assumes that both Strings are already in normalized form.
The value produced by a
&& or
|| operator is not necessarily of type Boolean. The value produced will always be the value of one of the two
operand expressions.
The grammar for a
ConditionalExpression in ECMAScript is slightly different from that in C and Java, which each allow the second subexpression to be an
Expression but restrict the third expression to be a
ConditionalExpression. The motivation for this difference in ECMAScript is to allow an assignment expression to be governed by either arm
of a conditional and to eliminate the confusing and fairly useless case of a comma expression as the centre expression.
When an assignment occurs within
strict mode code, it is an runtime error if
lref in step 1.f of the first algorithm or step 7 of the second algorithm it is an unresolvable reference. If
it is, a
ReferenceError exception is thrown. The
LeftHandSideExpression also may not be a reference to a data property with the attribute value {[[Writable]]:
false}, to an accessor property with the attribute value {[[Set]]:
undefined}, nor to a non-existent property of an object for which the
IsExtensible predicate returns the value
false. In these cases a
TypeError exception is thrown.
Left to right evaluation order is maintained by evaluating a
DestructuringAssignmentTarget that is not a destructuring pattern prior to accessing the iterator or evaluating the
Initializer.
The value of a
StatementList is the value of the last value producing item in the
StatementList. For example, the following calls to the
eval function all return the value 1:
When a
Block or
CaseBlock production is evaluated a new declarative
Environment Record is created and bindings for each block scoped variable, constant, function, generator function, or class declared in
the block are instantiated in the
Environment Record.
BlockDeclarationInstantiation is performed as follows using arguments
code and
env.
code is the grammar production corresponding to the body of the block.
env is the
Lexical Environment in which bindings are to be created.
If a
VariableDeclaration is nested within a with statement and the
BindingIdentifier in the
VariableDeclaration is the same as a property name of the binding object of the with statement's object
Environment Record, then step 6 will assign
value to the property instead of assigning to the VariableEnvironment binding of the
Identifier.
When
undefined is passed for
environment it indicates that a
PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict
functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of
multiple parameters with the same name.
When
undefined is passed for
environment it indicates that a
PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict
functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of
multiple parameters with the same name.
With parameters
value,
environment, and
propertyName.
Note
When
undefined is passed for
environment it indicates that a
PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict
functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of
multiple parameters with the same name.
Each
else for which the choice of associated
if is ambiguous shall be associated with the nearest possible
if that would otherwise have no corresponding
else.
undefined is passed for
environment to indicate that a
PutValue operation should be used to assign the initialization value. This is the case for
var statements and the formal parameter lists of some non-strict functions (see
9.2.12). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.
The abstract operation ForIn/OfHeadEvaluation is called with arguments
TDZnames,
expr, and
iterationKind. The value of
iterationKind is either
enumerate or
iterate.
The abstract operation ForIn/OfBodyEvaluation is called with arguments
lhs,
stmt,
iterator,
lhsKind, and
labelSet. The value of
lhsKind is either
assignment,
varBinding or
lexicalBinding.
Return an Iterator object (
25.1.1.2) whose
next method iterates over all the String-valued keys of enumerable properties of
O. The iterator object is never directly accessible to ECMAScript code. The mechanics and order of enumerating
the properties is not specified but must conform to the rules specified below.
The iterator's
throw and
return methods are
null and are never invoked. The iterator's
next method processes object properties to determine whether the property key should be returned as an iterator
value. Returned property keys do not include keys that are Symbols. Properties of the target object may be deleted
during enumeration. A property that is deleted before it is processed by the iterator's
next method is ignored. If new properties are added to the target object during enumeration, the newly added
properties are not guaranteed to be processed in the active enumeration. A property name will be returned by the iterator's
next method at most once in any enumeration.
Enumerating the properties of the target object includes enumerating properties of its prototype, and the prototype
of the prototype, and so on, recursively; but a property of a prototype is not processed if it has the same name as
a property that has already been processed by the iterator's
next method. The values of [[Enumerable]] attributes are not considered when determining if a property of a
prototype object has already been processed. The enumerable property names of prototype objects must be obtained by
invoking EnumerateObjectProperties passing the prototype object as the argument. EnumerateObjectProperties must obtain
the own property keys of the target object by calling its [[OwnPropertyKeys]] internal method. Property attributes
of the target object must be obtained by calling its [[GetOwnProperty]] internal method.
Note
The following is an informative definition of an ECMAScript generator function that conforms to these rules:
function* EnumerateObjectProperties(obj) {
let visited = newSet;
for (let key ofReflect.ownKeys(obj)) {
if (typeof key === "string") {
let desc = Reflect.getOwnPropertyDescriptor(obj, key);
if (desc && !visited.has(key)) {
visited.add(key);
if (desc.enumerable) yield key;
}
}
}
let proto = Reflect.getPrototypeOf(obj)
if (proto === null) return;
for (let protoName of EnumerateObjectProperties(proto)) {
if (!visited.has(protoName)) yield protoName;
}
}
It is a Syntax Error if this production is not nested, directly or indirectly (but not crossing function boundaries), within
an
IterationStatement or a
SwitchStatement.
A
return statement causes a function to cease execution and return a value to the caller. If
Expression is omitted, the return value is
undefined. Otherwise, the return value is the value of
Expression.
The
with statement adds an object
Environment Record for a computed object to the lexical environment of the
running execution context. It then executes a statement using this augmented lexical environment. Finally, it restores the original lexical environment.
No matter how control leaves the embedded
Statement, whether normally or by some form of
abrupt completion or exception, the LexicalEnvironment is always restored to its former state.
CaseSelectorEvaluation does not execute the associated
StatementList. It simply evaluates the
Expression and returns the value, which the
CaseBlock algorithm uses to determine which
StatementList to start executing.
A
Statement may be prefixed by a label. Labelled statements are only used in conjunction with labelled
break and
continue statements. ECMAScript has no
goto statement. A
Statement can be part of a
LabelledStatement, which itself can be part of a
LabelledStatement, and so on. The labels introduced this way are collectively referred to as the “current label set” when describing
the semantics of individual statements.
The
try statement encloses a block of code in which an exceptional condition can occur, such as a runtime error or
a
throw statement. The
catch clause provides the exception-handling code. When a catch clause catches an exception, its
CatchParameter is bound to that exception.
14.1.1Directive Prologues and the Use Strict Directive(指令序言和使用严格指令)
#
A
Directive Prologue is the longest sequence of
ExpressionStatement productions occurring as the initial
StatementListItem or
ModuleItem productions of a
FunctionBody, a
ScriptBody, or a
ModuleBody and where each
ExpressionStatement in the sequence consists entirely of a
StringLiteral token followed by a semicolon. The semicolon may appear explicitly or may be inserted by automatic semicolon insertion.
A Directive Prologue may be an empty sequence.(指令序言是作为FunctionBody，ScriptBody或ModuleBody的初始声明列表项或模块项产生而出现的最长的表达式语句产生序列，并且序列中的每个表达式语句完全由字符串字面值标记后跟分号组成。
分号可能会显式出现，或者可能通过自动分号插入。 指令序言可能是一个空的序列。)
Multiple occurrences of the same
BindingIdentifier in a
FormalParameterList is only allowed for functions and generator functions which have simple parameter lists and which are not defined in
strict mode code.
The ExpectedArgumentCount of a
FormalParameterList is the number of
FormalParameters to the left of either the rest parameter or the first
FormalParameter with an Initializer. A
FormalParameter without an initializer is allowed after the first parameter with an initializer but such parameters are considered
to be optional with
undefined as their default value.
14.1.10Static Semantics: IsAnonymousFunctionDefinition (
production )(匿名函数定义)
#
The abstract operation IsAnonymousFunctionDefinition determines if its argument is a function definition that does not
bind a name. The argument
production is the result of parsing an
AssignmentExpression or
Initializer. The following steps are taken:
If IsFunctionDefinition of
production is
false, return
false.
Let
hasName be the result of HasName of
production.
If ContainsExpression of
BindingElement is
false, return the result of performing IteratorBindingInitialization for
BindingElement using
iteratorRecord and
environment as the arguments.
If ContainsExpression of
BindingRestElement is
false, return the result of performing IteratorBindingInitialization for
BindingRestElement using
iteratorRecord and
environment as the arguments.
A
prototype property is automatically created for every function defined using a
FunctionDeclaration or
FunctionExpression, to allow for the possibility that the function will be used as a constructor.
Normally, Contains does not look inside most function forms. However, Contains is used to detect
new.target,
this, and
super usage within an
ArrowFunction.
When
undefined is passed for
environment it indicates that a
PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict
functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple
parameters with the same name.
An
ArrowFunction does not define local bindings for
arguments,
super,
this, or
new.target. Any reference to
arguments,
super,
this, or
new.target within an
ArrowFunction must resolve to a binding in a lexically enclosing environment. Typically this will be the Function Environment of
an immediately enclosing function. Even though an
ArrowFunction may contain references to
super, the function object created in step 4 is not made into a method by performing
MakeMethod. An
ArrowFunction that references
super is always contained within a non-
ArrowFunction and the necessary state to implement
super is accessible via the
scope that is captured by the function object of the
ArrowFunction.
注解： function a(...list) { 'use strict' console.log(list); } 在chrome中会报语法错误:Uncaught SyntaxError: Illegal 'use strict'
directive in function with non-simple parameter list
Let
innerResult be ?
Call(
throw,
iterator, «
received.[[Value]] »).
NOTE: Exceptions from the inner iterator
throw method are propagated. Normal completions from an inner
throw method are processed similarly to an inner
next.
If
Type(
innerResult) is not Object, throw a
TypeError exception.
NOTE: If
iterator does not have a
throw method, this throw is going to terminate the
yield* loop. But first we need to give
iterator a chance to clean up.
如果ClassElementList的PrototypePropertyNameList包含多个“构造函数”，则是一个语法错误。
class test { constructor () {} constructor (arg1) {} } Uncaught SyntaxError: A class may only have one constructor
如果MethodDefinition的PropName不是“构造函数”，并且MethodDefinition的HasDirectSuper为true，那么这是一个语法错误。 意思是非构造函数不能有super语句
class ClassA { } class ClassB extends ClassA { say () { super(); } } Uncaught SyntaxError: 'super' keyword unexpected here
如果MethodDefinition的PropName是“构造函数”，并且MethodDefinition的SpecialMethod是true，那么它是一个语法错误。 意思是构造函数不能是generator函数，get函数，set函数，否则报错
class ClassC { * constructor() {} } Uncaught SyntaxError: Class constructor may not be a generator
如果MethodDefinition的HasDirectSuper为true，那么这是一个语法错误。 静态方法不能有super语句
class ClassSt extends ClassA { constructor() {} static say () { super(); } } Uncaught SyntaxError: 'super' keyword unexpected
here
如果MethodDefinition的PropName是“prototype”，那么它是一个语法错误。
class ClassP { constructor() {} static prototype () { } } Classes may not have static property named prototype
Let
constructorInfo be the result of performing DefineMethod for
constructor with arguments
proto and
constructorParent as the optional
functionPrototype argument.
Return the result of HasProductionInTailPosition of
body with argument
nonterminal.
Note
Tail Position calls are only defined in
strict mode code because of a common non-standard language extension (see
9.2.7) that enables observation of the chain of caller contexts.
nonterminal is a parsed grammar production that represents a specific range of source text. When the following
algorithms compare
nonterminal to other grammar symbols they are testing whether the same source text was matched by both symbols.
A potential tail position call that is immediately followed by return
GetValue of the call result is also a possible tail position call. Function calls cannot return reference values, so such a
GetValue operation will always returns the same value as the actual function call result.
Assert:
leafContext has no further use. It will never be activated as the
running execution context.
A tail position call must either release any transient internal resources associated with the currently executing function
execution context before invoking the target function or reuse those resources in support of the target function.
For example, a tail position call should only grow an implementation's activation record stack by the amount that the
size of the target function's activation record exceeds the size of the calling function's activation record. If the
target function's activation record is smaller, then the total size of the stack should decrease.
It is a Syntax Error if
StatementList Contains
super unless the source code containing
super is eval code that is being processed by a
direct eval that is contained in function code that is not the function code of an
ArrowFunction.
It is a Syntax Error if
StatementList Contains
NewTarget unless the source code containing
NewTarget is eval code that is being processed by a
direct eval that is contained in function code that is not the function code of an
ArrowFunction.
The abstract operation ParseScript with arguments
sourceText,
realm, and
hostDefined creates a
Script Record based upon the result of parsing
sourceText as a
Script. ParseScript performs the following steps:
Assert:
sourceText is an ECMAScript source text (see clause
10).
Parse
sourceText using
Script as the goal symbol and analyze the parse result for any Early Error conditions. If the parse was successful and no
early errors were found, let
body be the resulting parse tree. Otherwise, let
body be a
List of one or more
SyntaxError or
ReferenceError objects representing the parsing errors and/or early errors. Parsing and
early error detection may be interweaved in an implementation dependent manner. If more than one parsing error or
early error is present, the number and ordering of error objects in the list is implementation dependent, but at least one must
be present.
An implementation may parse script source text and analyze it for Early Error conditions prior to evaluation of ParseScript
for that script source text. However, the reporting of any errors must be deferred until the point where this specification
actually performs ParseScript upon that source text.
When an
execution context is established for evaluating scripts, declarations are instantiated in the current
global environment. Each global binding declared in the code is instantiated.
GlobalDeclarationInstantiation is performed as follows using arguments
script and
env.
script is the
ScriptBody for which the
execution context is being established.
env is the global lexical environment in which bindings are to be created.
If
vn is not an element of
declaredFunctionNames, then
Let
vnDefinable be ?
envRec.CanDeclareGlobalVar(
vn).
If
vnDefinable is
false, throw a
TypeError exception.
If
vn is not an element of
declaredVarNames, then
Append
vn to
declaredVarNames.
NOTE: No abnormal terminations occur after this algorithm step if the
global object is an ordinary object. However, if the
global object is a Proxy exotic object it may exhibit behaviours that cause abnormal terminations in some of the following steps.
NOTE: Annex
B.3.3.2 adds additional steps at this point.
Let
lexDeclarations be the LexicallyScopedDeclarations of
script.
For each element
d in
lexDeclarations do
NOTE Lexically declared names are only instantiated here but not initialized.
Early errors specified in
15.1.1 prevent name conflicts between function/var declarations and let/const/class declarations as well as redeclaration
of let/const/class bindings for declaration contained within a single
Script. However, such conflicts and redeclarations that span more than one
Script are detected as runtime errors during GlobalDeclarationInstantiation. If any such errors are detected, no bindings
are instantiated for the script. However, if the
global object is defined using Proxy exotic objects then the runtime tests for conflicting declarations may be unreliable resulting
in an
abrupt completion and some global declarations not being instantiated. If this occurs, the code for the
Script is not evaluated.
Unlike explicit var or function declarations, properties that are directly created on the
global object result in global bindings that may be shadowed by let/const/class declarations.
It is a Syntax Error if the LexicallyDeclaredNames of
ModuleItemList contains any duplicate entries.
It is a Syntax Error if any element of the LexicallyDeclaredNames of
ModuleItemList also occurs in the VarDeclaredNames of
ModuleItemList.
It is a Syntax Error if the ExportedNames of
ModuleItemList contains any duplicate entries.
It is a Syntax Error if any element of the ExportedBindings of
ModuleItemList does not also occur in either the VarDeclaredNames of
ModuleItemList, or the LexicallyDeclaredNames of
ModuleItemList.
The duplicate ExportedNames rule implies that multiple
export defaultExportDeclaration items within a
ModuleBody is a Syntax Error. Additional error conditions relating to conflicting or duplicate declarations are checked during
module linking prior to evaluation of a
Module. If any such errors are detected the
Module is not evaluated.
The abstract operation ImportedLocalNames with argument
importEntries creates a
List of all of the local name bindings defined by a
List of ImportEntry Records (see
Table 40). ImportedLocalNames performs the following steps:
A
Module Record encapsulates structural information about the imports and exports of a single module. This information
is used to link the imports and exports of sets of connected modules. A Module Record includes four fields that are
only used when evaluating a module.
For specification purposes Module Record values are values of the
Record specification type and can be thought of as existing in a simple object-oriented hierarchy where Module Record is an
abstract class with concrete subclasses. This specification only defines a single Module Record concrete subclass named
Source Text Module Record. Other specifications and implementations may define additional Module Record subclasses corresponding to alternative
module definition facilities that they defined.
Module Record defines the fields listed in
Table 37. All Module Definition subclasses include at least those fields. Module Record also defines the abstract method list
in
Table 38. All Module definition subclasses must provide concrete implementations of these abstract methods.
Return the binding of a name exported by this module. Bindings are represented by a
Record of the form {[[Module]]: Module Record, [[BindingName]]: String}
ModuleDeclarationInstantiation()
Transitively resolve all module dependencies and create a module
Environment Record for the module.
ModuleEvaluation()
Do nothing if this module has already been evaluated. Otherwise, transitively evaluate all module dependences
of this module and then evaluate this module.
ModuleDeclarationInstantiation must be completed prior to invoking this method.
A
Source Text Module Record is used to represent information about a module that was defined from ECMAScript source
text (10) that was parsed using the goal symbol
Module. Its fields contain digested information about the names that are imported by the module and its concrete methods
use this digest to link, instantiate, and evaluate the module.
In addition to the fields, defined in
Table 37, Source Text Module Records have the additional fields listed in
Table 39. Each of these fields initially has the value
undefined.
Table 39: Additional Fields of Source Text Module Records
Field Name
Value Type
Meaning
[[ECMAScriptCode]]
a parse result
The result of parsing the source text of this module using
Module as the goal symbol.
A
List of all the
ModuleSpecifier strings used by the module represented by this record to request the importation of a module. The
List is source code occurrence ordered.
A
List of ExportEntry records derived from the code of this module that correspond to export * declarations that occur
within the module.
An ImportEntry
Record is a
Record that digests information about a single declarative import. Each ImportEntry
Record has the fields defined in
Table 40:
The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. The value
"*" indicates that the import request is for the target module's namespace object.
[[LocalName]]
String
The name that is used to locally access the imported value from within the importing module.
Note 1
Table 41 gives examples of ImportEntry records fields used to represent the syntactic import forms:
Table 41 (Informative): Import Forms Mappings to ImportEntry Records
An ExportEntry
Record is a
Record that digests information about a single declarative export. Each ExportEntry
Record has the fields defined in
Table 42:
The name under which the desired binding is exported by the module identified by [[ModuleRequest]].
null if the
ExportDeclaration does not have a
ModuleSpecifier.
"*" indicates that the export request is for all exported bindings.
[[LocalName]]
String | null
The name that is used to locally access the exported value from within the importing module.
null if the exported value is not locally accessible from within the module.
Note 2
Table 43 gives examples of the ExportEntry record fields used to represent the syntactic export forms:
Table 43 (Informative): Export Forms Mappings to ExportEntry Records
Export Statement Form
[[ExportName]]
[[ModuleRequest]]
[[ImportName]]
[[LocalName]]
export var v;
"v"
null
null
"v"
export default function f(){}
"default"
null
null
"f"
export default function(){}
"default"
null
null
"*default*"
export default 42;
"default"
null
null
"*default*"
export {x};
"x"
null
null
"x"
export {v as x};
"x"
null
null
"v"
export {x} from "mod";
"x"
"mod"
"x"
null
export {v as x} from "mod";
"x"
"mod"
"v"
null
export * from "mod";
null
"mod"
"*"
null
The following definitions specify the required concrete methods and other abstract operations for Source Text Module
Records
The abstract operation ParseModule with arguments
sourceText,
realm, and
hostDefined creates a
Source Text Module Record based upon the result of parsing
sourceText as a
Module. ParseModule performs the following steps:
Assert:
sourceText is an ECMAScript source text (see clause
10).
Parse
sourceText using
Module as the goal symbol and analyze the parse result for any Early Error conditions. If the parse was successful and
no early errors were found, let
body be the resulting parse tree. Otherwise, let
body be a
List of one or more
SyntaxError or
ReferenceError objects representing the parsing errors and/or early errors. Parsing and
early error detection may be interweaved in an implementation dependent manner. If more than one parsing error or
early error is present, the number and ordering of error objects in the list is implementation dependent, but at least one
must be present.
If
ee.[[LocalName]] is not an element of
importedBoundNames, then
Append
ee to
localExportEntries.
Else,
Let
ie be the element of
importEntries whose [[LocalName]] is the same as
ee.[[LocalName]].
If
ie.[[ImportName]] is
"*", then
Assert: this is a re-export of an imported module namespace object.
Append
ee to
localExportEntries.
Else, this is a re-export of a single name
Append to
indirectExportEntries the
Record {[[ModuleRequest]]:
ie.[[ModuleRequest]], [[ImportName]]:
ie.[[ImportName]], [[LocalName]]:
null, [[ExportName]]:
ee.[[ExportName]] }.
An implementation may parse module source text and analyze it for Early Error conditions prior to the evaluation of
ParseModule for that module source text. However, the reporting of any errors must be deferred until the point where
this specification actually performs ParseModule upon that source text.
The ResolveExport concrete method of a
Source Text Module Record with arguments
exportName,
resolveSet, and
exportStarSet performs the following steps:
Let
resolution be ?
importedModule.ResolveExport(
exportName,
resolveSet,
exportStarSet).
If
resolution is
"ambiguous", return
"ambiguous".
If
resolution is not
null, then
If
starResolution is
null, let
starResolution be
resolution.
Else,
Assert: there is more than one
* import that includes the requested name.
If
resolution.[[Module]] and
starResolution.[[Module]] are not the same
Module Record or
SameValue(
resolution.[[BindingName]],
starResolution.[[BindingName]]) is
false, return
"ambiguous".
Return
starResolution.
Note
ResolveExport attempts to resolve an imported binding to the actual defining module and local binding name. The defining
module may be the module represented by the
Module Record this method was invoked on or some other module that is imported by that module. The parameter
resolveSet is use to detect unresolved circular import/export paths. If a pair consisting of specific
Module Record and
exportName is reached that is already in
resolveSet, an import circularity has been encountered. Before recursively calling ResolveExport, a pair consisting
of
module and
exportName is added to
resolveSet.
If a defining module is found a
Record {[[Module]], [[BindingName]]} is returned. This record identifies the resolved binding of the originally requested
export. If no definition was found or the request is found to be circular,
null is returned. If the request is found to be ambiguous, the string
"ambiguous" is returned.
For each String
required that is an element of
module.[[RequestedModules]] do,
NOTE: Before instantiating a module, all of the modules it requested must be available. An implementation may perform
this test at any time prior to this point.
HostResolveImportedModule is an implementation defined abstract operation that provides the concrete
Module Record subclass instance that corresponds to the
ModuleSpecifier String,
specifier, occurring within the context of the module represented by the
Module RecordreferencingModule.
The implementation of HostResolveImportedModule must conform to the following requirements:
The normal return value must be an instance of a concrete subclass of
Module Record.
If a
Module Record corresponding to the pair
referencingModule,
specifier does not exist or cannot be created, an exception must be thrown.
This operation must be idempotent if it completes normally. Each time it is called with a specific
referencingModule,
specifier pair as arguments it must return the same
Module Record instance.
Multiple different
referencingModule,
specifier pairs may map to the same
Module Record instance. The actual mapping semantic is implementation defined but typically a normalization process is applied to
specifier as part of the mapping process. A typical normalization process would include actions such as alphabetic
case folding and expansion of relative and abbreviated path specifiers.
An implementation may parse a
sourceText as a
Module, analyze it for Early Error conditions, and instantiate it prior to the execution of the TopLevelModuleEvaluationJob
for that
sourceText. An implementation may also resolve, pre-parse and pre-analyze, and pre-instantiate module dependencies
of
sourceText. However, the reporting of any errors detected by these actions must be deferred until the TopLevelModuleEvaluationJob
is actually executed.
For each
IdentifierNamen in ReferencedBindings of
ExportClause: It is a Syntax Error if StringValue of
n is a
ReservedWord or if the StringValue of
n is one of:
"implements",
"interface",
"let",
"package",
"private",
"protected",
"public", or
"static".
Return a new
List containing the
Record {[[ModuleRequest]]:
module, [[ImportName]]:
importName, [[LocalName]]:
localName, [[ExportName]]:
sourceName }.
Let
sourceName be the StringValue of the first
IdentifierName.
Let
exportName be the StringValue of the second
IdentifierName.
If
module is
null, then
Let
localName be
sourceName.
Let
importName be
null.
Else,
Let
localName be
null.
Let
importName be
sourceName.
Return a new
List containing the
Record {[[ModuleRequest]]:
module, [[ImportName]]:
importName, [[LocalName]]:
localName, [[ExportName]]:
exportName }.
It is not necessary to treat
export defaultAssignmentExpression as a constant declaration because there is no syntax that permits assignment to the internal bound name used to reference
a module's default object.
An implementation must report most errors at the time the relevant ECMAScript language construct is evaluated. An
early error is an error that can be detected and reported prior to the evaluation of any construct in the
Script containing the error. The presence of an early error prevents the evaluation of the construct. An implementation must
report early errors in a
Script as part of parsing that
Script in
ParseScript. Early errors in a
Module are reported at the point when the
Module would be evaluated and the
Module is never initialized. Early errors in
eval code are reported at the time
eval is called and prevent evaluation of the
eval code. All errors that are not early errors are runtime errors.
An implementation must report as an early error any occurrence of a condition that is listed in a “Static Semantics: Early
Errors” subclause of this specification.
An implementation shall not treat other kinds of errors as early errors even if the compiler can prove that a construct
cannot execute without error under any circumstances. An implementation may issue an early warning in such a case, but
it should not report the error until the relevant construct is actually executed.
An implementation shall report all errors as specified, except for the following:
Except as restricted in
16.2, an implementation may extend
Script syntax,
Module syntax, and regular expression pattern or flag syntax. To permit this, all operations (such as calling
eval, using a regular expression literal, or using the
Function or
RegExp constructor) that are allowed to throw
SyntaxError are permitted to exhibit implementation-defined behaviour instead of throwing
SyntaxError when they encounter an implementation-defined extension to the script syntax or regular expression
pattern or flag syntax.
Except as restricted in
16.2, an implementation may provide additional types, values, objects, properties, and functions beyond those described in
this specification. This may cause constructs (such as looking up a variable in the global scope) to have implementation-defined
behaviour instead of throwing an error (such as
ReferenceError).
An implementation may define behaviour other than throwing
RangeError for
toFixed,
toExponential, and
toPrecision when the
fractionDigits or
precision argument is outside the specified range.
HostReportErrors is an implementation-defined abstract operation that allows host environments to report parsing errors,
early errors, and runtime errors.
An implementation of HostReportErrors must complete normally in all cases. The default implementation of HostReportErrors
is to do nothing.
Note
errorList will be a
List of ECMAScript language values. If the errors are parsing errors or early errors, these will always be
SyntaxError or
ReferenceError objects. Runtime errors, however, can be any ECMAScript value.
An implementation must not extend this specification in the following ways:
Other than as defined in this specification, ECMAScript Function objects defined using syntactic constructors in
strict mode code must not be created with own properties named
"caller" or
"arguments" other than those that are created by applying the
AddRestrictedFunctionProperties abstract operation to the function. Such own properties also must not be created for function objects defined using
an
ArrowFunction,
MethodDefinition,
GeneratorDeclaration,
GeneratorExpression,
ClassDeclaration, or
ClassExpression regardless of whether the definition is contained in
strict mode code. Built-in functions, strict mode functions created using the
Function constructor, generator functions created using the
Generator constructor, and functions created using the
bind method also must not be created with such own properties.
If an implementation extends non-strict or built-in function objects with an own property named
"caller" the value of that property, as observed using [[Get]] or [[GetOwnProperty]], must not be a strict function
object. If it is an accessor property, the function that is the value of the property's [[Get]] attribute must never
return a strict function when called.
The behaviour of the following methods must not be extended except as specified in ECMA-402:
Object.prototype.toLocaleString,
Array.prototype.toLocaleString,
Number.prototype.toLocaleString,
Date.prototype.toLocaleDateString,
Date.prototype.toLocaleString,
Date.prototype.toLocaleTimeString,
String.prototype.localeCompare,
%TypedArray%.prototype.toLocaleString.
The RegExp pattern grammars in
21.2.1 and
B.1.4 must not be extended to recognize any of the source characters A-Z or a-z as
IdentityEscape[U] when the U grammar parameter is present.
The Syntactic Grammar must not be extended in any manner that allows the token
: to immediate follow source text that matches the
BindingIdentifier nonterminal symbol.
There are certain built-in objects available whenever an ECMAScript
Script or
Module begins execution. One, the
global object, is part of the lexical environment of the executing program. Others are accessible as initial properties of the
global object or indirectly as properties of accessible built-in objects.
Unless specified otherwise, a built-in object that is callable as a function is a built-in Function object with the characteristics
described in
9.3. Unless specified otherwise, the [[Extensible]] internal slot of a built-in object initially has the value
true. Every built-in Function object has a [[Realm]] internal slot whose value is the
Realm Record of the
realm for which the object was initially created.
Many built-in objects are functions: they can be invoked with arguments. Some of them furthermore are constructors: they
are functions intended for use with the
new operator. For each built-in function, this specification describes the arguments required by that function
and the properties of that function object. For each built-in constructor, this specification furthermore describes properties
of the prototype object of that constructor and properties of specific object instances returned by a
new expression that invokes that constructor.
Unless otherwise specified in the description of a particular function, if a built-in function or constructor is given
fewer arguments than the function is specified to require, the function or constructor shall behave exactly as if it had
been given sufficient additional arguments, each such argument being the
undefined value. Such missing arguments are considered to be “not present” and may be identified in that manner
by specification algorithms. In the description of a particular function, the terms “
this value” and “NewTarget” have the meanings given in
9.3.
Unless otherwise specified in the description of a particular function, if a built-in function or constructor described
is given more arguments than the function is specified to allow, the extra arguments are evaluated by the call and then
ignored by the function. However, an implementation may define implementation specific behaviour relating to such arguments
as long as the behaviour is not the throwing of a
TypeError exception that is predicated simply on the presence of an extra argument.
Note 1
Implementations that add additional capabilities to the set of built-in functions are encouraged to do so by adding new
functions rather than adding new parameters to existing functions.
Unless otherwise specified every built-in function and every built-in constructor has the Function prototype object, which
is the initial value of the expression
Function.prototype (
19.2.3), as the value of its [[Prototype]] internal slot.
Unless otherwise specified every built-in prototype object has the Object prototype object, which is the initial value
of the expression
Object.prototype (
19.1.3), as the value of its [[Prototype]] internal slot, except the Object prototype object itself.
Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless
otherwise specified in the description of a particular function.
Unless otherwise specified, each built-in function defined in this specification is created as if by calling the
CreateBuiltinFunction abstract operation (
9.3.3).
Every built-in Function object, including constructors, has a
length property whose value is an integer. Unless otherwise specified, this value is equal to the largest number
of named arguments shown in the subclause headings for the function description. Optional parameters (which are indicated
with brackets:
[]) or rest parameters (which are shown using the form «...name») are not included in the default argument count.
Note 2
For example, the function object that is the initial value of the
map property of the Array prototype object is described under the subclause heading «Array.prototype.map (callbackFn
[ , thisArg])» which shows the two named arguments callbackFn and thisArg, the latter being optional; therefore the value
of the
length property of that Function object is
1.
Unless otherwise specified, the
length property of a built-in Function object has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
Every built-in Function object, including constructors, that is not identified as an anonymous function has a
name property whose value is a String. Unless otherwise specified, this value is the name that is given to the
function in this specification. For functions that are specified as properties of objects, the name value is the property
name string used to access the function. Functions that are specified as get or set accessor functions of built-in properties
have
"get " or
"set " prepended to the property name string. The value of the
name property is explicitly specified for each built-in functions whose property key is a Symbol value.
Unless otherwise specified, the
name property of a built-in Function object, if it exists, has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
Every other data property described in clauses 18 through 26 and in Annex
B.2 has the attributes { [[Writable]]:
true, [[Enumerable]]:
false, [[Configurable]]:
true } unless otherwise specified.
Every accessor property described in clauses 18 through 26 and in Annex
B.2 has the attributes { [[Enumerable]]:
false, [[Configurable]]:
true } unless otherwise specified. If only a get accessor function is described, the set accessor function is
the default value,
undefined. If only a set accessor is described the get accessor is the default value,
undefined.
The
global object does not have a [[Construct]] internal method; it is not possible to use the
global object as a constructor with the
new operator.
The
global object does not have a [[Call]] internal method; it is not possible to invoke the
global object as a function.
The value of the [[Prototype]] internal slot of the
global object is implementation-dependent.
In addition to the properties defined in this specification the
global object may have additional host defined properties. This may include a property whose value is the
global object itself; for example, in the HTML document object model the
window property of the
global object is the
global object itself.
The value of
undefined is
undefined (see
6.1.1). This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
Let
script be the ECMAScript code that is the result of parsing
x, interpreted as UTF-16 encoded Unicode text as described in
6.1.4, for the goal symbol
Script. If the parse fails, throw a
SyntaxError exception. If any early errors are detected, throw a
SyntaxError or a
ReferenceError exception, depending on the type of the error (but see also clause
16). Parsing and
early error detection may be interweaved in an implementation dependent manner.
If
script Contains
ScriptBody is
false, return
undefined.
The eval code cannot instantiate variable or function bindings in the variable environment of the calling context that
invoked the eval if the calling context is evaluating formal parameter initializers or if either the code of the calling
context or the eval code is strict code. Instead such bindings are instantiated in a new VariableEnvironment that
is only accessible to the eval code. Bindings introduced by
let,
const, or
class declarations are always instantiated in a new LexicalEnvironment.
Let
vnDefinable be ?
varEnvRec.CanDeclareGlobalVar(
vn).
If
vnDefinable is
false, throw a
TypeError exception.
If
vn is not an element of
declaredVarNames, then
Append
vn to
declaredVarNames.
NOTE: No abnormal terminations occur after this algorithm step unless
varEnvRec is a global
Environment Record and the
global object is a Proxy exotic object.
Let
lexDeclarations be the LexicallyScopedDeclarations of
body.
For each element
d in
lexDeclarations do
NOTE Lexically declared names are only instantiated here but not initialized.
The
isFinite function is the
%isFinite% intrinsic object. When the
isFinite function is called with one argument
number, the following steps are taken:
A reliable way for ECMAScript code to test if a value
X is a
NaN is an expression of the form
X !== X. The result will be
true if and only if
X is a
NaN.
The
parseFloat function produces a Number value dictated by interpretation of the contents of the
string argument as a decimal literal.
The
parseFloat function is the
%parseFloat% intrinsic object. When the
parseFloat function is called with one argument
string, the following steps are taken:
Let
trimmedString be a substring of
inputString consisting of the leftmost code unit that is not a
StrWhiteSpaceChar and all code units to the right of that code unit. (In other words, remove leading white space.) If
inputString does not contain any such code units, let
trimmedString be the empty string.
If neither
trimmedString nor any prefix of
trimmedString satisfies the syntax of a
StrDecimalLiteral (see
7.1.3.1), return
NaN.
Let
numberString be the longest prefix of
trimmedString, which might be
trimmedString itself, that satisfies the syntax of a
StrDecimalLiteral.
Let
mathFloat be MV of
numberString.
If
mathFloat=0, then
If the first code unit of
trimmedString is
"-", return
-0.
Return
+0.
Return the Number value for
mathFloat.
Note
parseFloat may interpret only a leading portion of
string as a Number value; it ignores any code units that cannot be interpreted as part of the notation of an
decimal literal, and no indication is given that any such code units were ignored.
The
parseInt function produces an integer value dictated by interpretation of the contents of the
string argument according to the specified
radix. Leading white space in
string is ignored. If
radix is
undefined or 0, it is assumed to be 10 except when the number begins with the code unit pairs
0x or
0X, in which case a radix of 16 is assumed. If
radix is 16, the number may also optionally begin with the code unit pairs
0x or
0X.
The
parseInt function is the
%parseInt% intrinsic object. When the
parseInt function is called, the following steps are taken:
Let
S be a newly created substring of
inputString consisting of the first code unit that is not a
StrWhiteSpaceChar and all code units following that code unit. (In other words, remove leading white space.) If
inputString does not contain any such code unit, let
S be the empty string.
Let
sign be 1.
If
S is not empty and the first code unit of
S is 0x002D (HYPHEN-MINUS), let
sign be -1.
If
S is not empty and the first code unit of
S is 0x002B (PLUS SIGN) or 0x002D (HYPHEN-MINUS), remove the first code unit from
S.
If the length of
S is at least 2 and the first two code units of
S are either
"0x" or
"0X", remove the first two code units from
S and let
R be 16.
If
S contains a code unit that is not a radix-
R digit, let
Z be the substring of
S consisting of all code units before the first such code unit; otherwise, let
Z be
S.
If
Z is empty, return
NaN.
Let
mathInt be the mathematical integer value that is represented by
Z in radix-
R notation, using the letters
A-
Z and
a-
z for digits with values 10 through 35. (However, if
R is 10 and
Z contains more than 20 significant digits, every significant digit after the 20th may be replaced by a 0 digit,
at the option of the implementation; and if
R is not 2, 4, 8, 10, 16, or 32, then
mathInt may be an implementation-dependent approximation to the mathematical integer value that is represented
by
Z in radix-
R notation.)
If
mathInt = 0, then
If
sign = -1, return
-0.
Return
+0.
Let
number be the Number value for
mathInt.
Return
sign ×
number.
Note
parseInt may interpret only a leading portion of
string as an integer value; it ignores any code units that cannot be interpreted as part of the notation of an
integer, and no indication is given that any such code units were ignored.
Uniform Resource Identifiers, or URIs, are Strings that identify resources (e.g. web pages or files) and transport protocols
by which to access them (e.g. HTTP or FTP) on the Internet. The ECMAScript language itself does not provide any support
for using URIs except for functions that encode and decode URIs as described in
18.2.6.2,
18.2.6.3,
18.2.6.4 and
18.2.6.5
Note
Many implementations of ECMAScript provide additional functions and methods that manipulate web pages; these functions
are beyond the scope of this standard.
A URI is composed of a sequence of components separated by component separators. The general form is:
Scheme:First/Second;Third?Fourth
where the italicized names represent components and “
:”, “
/”, “
;” and “
?” are reserved for use as separators. The
encodeURI and
decodeURI functions are intended to work with complete URIs; they assume that any reserved code units in the
URI are intended to have special meaning and so are not encoded. The
encodeURIComponent and
decodeURIComponent functions are intended to work with the individual component parts of a URI; they assume
that any reserved code units represent text and so must be encoded so that they are not interpreted as reserved code
units when the component is part of a complete URI.
The following lexical grammar specifies the form of encoded URIs.
The above syntax is based upon RFC 2396 and does not reflect changes introduced by the more recent RFC 3986.
Runtime Semantics
When a code unit to be included in a URI is not listed above or is not intended to have the special meaning sometimes
given to the reserved code units, that code unit must be encoded. The code unit is transformed into its UTF-8 encoding,
with surrogate pairs first converted from UTF-16 to the corresponding code point value. (Note that for code units in
the range [0,127] this results in a single octet with the same value.) The resulting sequence of octets is then transformed
into a String with each octet represented by an escape sequence of the form
"%xx".
The unescaping and decoding process is described by the abstract operation Decode taking two String arguments
string and
reservedSet.
Let
strLen be the number of code units in
string.
Let
R be the empty String.
Let
k be 0.
Repeat
If
k equals
strLen, return
R.
Let
C be the code unit at index
k within
string.
If
C is not
"%", then
Let
S be the String containing only the code unit
C.
Else
C is
"%",
Let
start be
k.
If
k + 2 is greater than or equal to
strLen, throw a
URIError exception.
If the code units at index (
k + 1) and (
k + 2) within
string do not represent hexadecimal digits, throw a
URIError exception.
Let
B be the 8-bit value represented by the two hexadecimal digits at index (
k + 1) and (
k + 2).
Increment
k by 2.
If the most significant bit in
B is 0, then
Let
C be the code unit with code unit value
B.
If
C is not in
reservedSet, then
Let
S be the String containing only the code unit
C.
Else
C is in
reservedSet,
Let
S be the substring of
string from index
start to index
k inclusive.
Else the most significant bit in
B is 1,
Let
n be the smallest nonnegative integer such that (
B <<
n) & 0x80 is equal to 0.
If
n equals 1 or
n is greater than 4, throw a
URIError exception.
Let
Octets be an array of 8-bit integers of size
n.
Put
B into
Octets at index 0.
If
k + (3 × (
n - 1)) is greater than or equal to
strLen, throw a
URIError exception.
Let
j be 1.
Repeat, while
j <
n
Increment
k by 1.
If the code unit at index
k within
string is not
"%", throw a
URIError exception.
If the code units at index (
k + 1) and (
k + 2) within
string do not represent hexadecimal digits, throw a
URIError exception.
Let
B be the 8-bit value represented by the two hexadecimal digits at index (
k + 1) and (
k + 2).
If the two most significant bits in
B are not 10, throw a
URIError exception.
Increment
k by 2.
Put
B into
Octets at index
j.
Increment
j by 1.
Let
V be the value obtained by applying the UTF-8 transformation to
Octets, that is, from an array of octets into a 21-bit value. If
Octets does not contain a valid UTF-8 encoding of a Unicode code point, throw a
URIError exception.
If
V < 0x10000, then
Let
C be the code unit
V.
If
C is not in
reservedSet, then
Let
S be the String containing only the code unit
C.
Else
C is in
reservedSet,
Let
S be the substring of
string from index
start to index
k inclusive.
Else
V ≥ 0x10000,
Let
L be (((
V - 0x10000) & 0x3FF) + 0xDC00).
Let
H be ((((
V - 0x10000) >> 10) & 0x3FF) + 0xD800).
Let
S be the String containing the two code units
H and
L.
Let
R be a new String value computed by concatenating the previous value of
R and
S.
Increase
k by 1.
Note
This syntax of Uniform Resource Identifiers is based upon RFC 2396 and does not reflect the more recent RFC 3986 which
replaces RFC 2396. A formal description and implementation of UTF-8 is given in RFC 3629.
In UTF-8, characters are encoded using sequences of 1 to 6 octets. The only octet of a sequence of one has the higher-order
bit set to 0, the remaining 7 bits being used to encode the character value. In a sequence of n octets, n>1, the
initial octet has the n higher-order bits set to 1, followed by a bit set to 0. The remaining bits of that octet
contain bits from the value of the character to be encoded. The following octets all have the higher-order bit set
to 1 and the following bit set to 0, leaving 6 bits in each to contain bits from the character to be encoded. The
possible UTF-8 encodings of ECMAScript characters are specified in
Table 44.
Table 44 (Informative): UTF-8 Encodings
Code Unit Value
Representation
1
st Octet
2
nd Octet
3
rd Octet
4
th Octet
0x0000 - 0x007F
00000000 0
zzzzzzz
0
zzzzzzz
0x0080 - 0x07FF
00000
yyy yyzzzzzz
110
yyyyy
10
zzzzzz
0x0800 - 0xD7FF
xxxxyyyy yyzzzzzz
1110
xxxx
10
yyyyyy
10
zzzzzz
0xD800 - 0xDBFF followed by
0xDC00 - 0xDFFF
110110
vv vvwwwwxx followed by
110111
yy yyzzzzzz
11110
uuu
10
uuwwww
10
xxyyyy
10
zzzzzz
0xD800 - 0xDBFF not followed by
0xDC00 - 0xDFFF
causes
URIError
0xDC00 - 0xDFFF
causes
URIError
0xE000 - 0xFFFF
xxxxyyyy yyzzzzzz
1110
xxxx
10
yyyyyy
10
zzzzzz
Where
uuuuu =
vvvv + 1
to account for the addition of 0x10000 as in Surrogates, section 3.8, of the Unicode Standard.
The range of code unit values 0xD800-0xDFFF is used to encode surrogate pairs; the above transformation combines a
UTF-16 surrogate pair into a UTF-32 representation and encodes the resulting 21-bit value in UTF-8. Decoding reconstructs
the surrogate pair.
RFC 3629 prohibits the decoding of invalid UTF-8 octet sequences. For example, the invalid sequence C0 80 must not
decode into the code unit 0x0000. Implementations of the Decode algorithm are required to throw a
URIError when encountering such invalid sequences.
The
decodeURI function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort
that might be introduced by the
encodeURI function is replaced with the UTF-16 encoding of the code points that it represents. Escape sequences
that could not have been introduced by
encodeURI are not replaced.
The
decodeURI function is the
%decodeURI% intrinsic object. When the
decodeURI function is called with one argument
encodedURI, the following steps are taken:
The
decodeURIComponent function computes a new version of a URI in which each escape sequence and UTF-8 encoding
of the sort that might be introduced by the
encodeURIComponent function is replaced with the UTF-16 encoding of the code points that it represents.
The
decodeURIComponent function is the
%decodeURIComponent% intrinsic object. When the
decodeURIComponent function is called with one argument
encodedURIComponent, the following steps are taken:
Let
componentString be ?
ToString(
encodedURIComponent).
The
encodeURI function computes a new version of a UTF-16 encoded (
6.1.4) URI in which each instance of certain code points is replaced by one, two, three, or four escape sequences representing
the UTF-8 encoding of the code points.
The
encodeURI function is the
%encodeURI% intrinsic object. When the
encodeURI function is called with one argument
uri, the following steps are taken:
The
encodeURIComponent function computes a new version of a UTF-16 encoded (
6.1.4) URI in which each instance of certain code points is replaced by one, two, three, or four escape sequences representing
the UTF-8 encoding of the code point.
The
encodeURIComponent function is the
%encodeURIComponent% intrinsic object. When the
encodeURIComponent function is called with one argument
uriComponent, the following steps are taken:
The Object constructor is the
%Object% intrinsic object and the initial value of the
Object property of the
global object. When called as a constructor it creates a new ordinary object. When
Object is called as a function rather than as a constructor, it performs a type conversion.
The
Object constructor is designed to be subclassable. It may be used as the value of an
extends clause of a class definition.
The
assign function is used to copy the values of all of the enumerable own properties from one or more source objects
to a
target object. When the
assign function is called, the following steps are taken:
19.1.2.3Object.defineProperties (
O,
Properties )
#
The
defineProperties function is used to add own properties and/or update the attributes of existing own properties
of an object. When the
defineProperties function is called, the following steps are taken:
19.1.2.4Object.defineProperty (
O,
P,
Attributes )
#
The
defineProperty function is used to add an own property and/or update the attributes of an existing own property
of an object. When the
defineProperty function is called, the following steps are taken:
If
Type(
O) is not Object, throw a
TypeError exception.
19.1.2.8.1Runtime Semantics: GetOwnPropertyKeys (
O,
Type )
#
The abstract operation GetOwnPropertyKeys is called with arguments
O and
Type where
O is an Object and
Type is one of the ECMAScript specification types String or Symbol. The following steps are taken:
If an implementation defines a specific order of enumeration for the for-in statement, the same order must be used for
the elements of the array returned in step 3.
The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1 in previous
editions of this specification will continue to be thrown even if the
this value is
undefined or
null.
The ordering of steps 1 and 2 preserves the behaviour specified by previous editions of this specification for the
case where
V is not an object and the
this value is
undefined or
null.
19.1.3.4Object.prototype.propertyIsEnumerable (
V )
#
When the
propertyIsEnumerable method is called with argument
V, the following steps are taken:
This method does not consider objects in the prototype chain.
Note 2
The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1 in previous
editions of this specification will continue to be thrown even if the
this value is
undefined or
null.
The optional parameters to this function are not used but are intended to correspond to the parameter pattern used by
ECMA-402
toLocalString functions. Implementations that do not include ECMA-402 support must not use those parameter positions
for other purposes.
Note 1
This function provides a generic
toLocaleString implementation for objects that have no locale-specific
toString behaviour.
Array,
Number,
Date, and
Typed Arrays provide their own locale-sensitive
toLocaleString methods.
Note 2
ECMA-402 intentionally does not provide an alternative to this default implementation.
If
Type(
tag) is not String, let
tag be
builtinTag.
Return the String that is the result of concatenating
"[object ",
tag, and
"]".
This function is the
%ObjProto_toString% intrinsic object.
Note
Historically, this function was occasionally used to access the String value of the [[Class]] internal slot that was
used in previous editions of this specification as a nominal type tag for various built-in objects. The above definition
of
toString preserves compatibility for legacy code that uses
toString as a test for those specific kinds of built-in objects. It does not provide a reliable type testing
mechanism for other kinds of built-in or program defined objects. In addition, programs can use @@toStringTag in ways
that will invalidate the reliability of such legacy type tests.
The Function constructor is the
%Function% intrinsic object and the initial value of the
Function property of the
global object. When
Function is called as a function rather than as a constructor, it creates and initializes a new Function object.
Thus the function call
Function(…) is equivalent to the object creation expression
new Function(…) with the same arguments.
The
Function constructor is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass constructors that intend to inherit the specified
Function behaviour must include a
super call to the
Function constructor to create and initialize a subclass instances with the internal slots necessary for built-in
function behaviour. All ECMAScript syntactic forms for defining function objects create instances of
Function. There is no syntactic means to create instances of
Function subclasses except for the built-in Generator Function subclass.
The last argument specifies the body (executable code) of a function; any preceding arguments specify formal parameters.
When the
Function function is called with some arguments
p1,
p2, … ,
pn,
body (where
n might be 0, that is, there are no “
p” arguments, and where
body might also not be provided), the following steps are taken:
It is permissible but not necessary to have one argument for each formal parameter to be specified. For example, all
three of the following expressions produce the same result:
The abstract operation CreateDynamicFunction is called with arguments
constructor,
newTarget,
kind, and
args.
constructor is the constructor function that is performing this action,
newTarget is the constructor that
new was initially applied to,
kind is either
"normal" or
"generator", and
args is a
List containing the actual argument values that were passed to
constructor. The following steps are taken:
If
newTarget is
undefined, let
newTarget be
constructor.
Let
parameters be the result of parsing
P, interpreted as UTF-16 encoded Unicode text as described in
6.1.4, using
parameterGoal as the goal symbol. Throw a
SyntaxError exception if the parse fails.
Let
body be the result of parsing
bodyText, interpreted as UTF-16 encoded Unicode text as described in
6.1.4, using
goal as the goal symbol. Throw a
SyntaxError exception if the parse fails.
If
bodyText is
strict mode code, then let
strict be
true, else let
strict be
false.
If any static semantics errors are detected for
parameters or
body, throw a
SyntaxError or a
ReferenceError exception, depending on the type of the error. If
strict is
true, the Early Error rules for
StrictFormalParameters:FormalParameters are applied. Parsing and
early error detection may be interweaved in an implementation dependent manner.
If ContainsUseStrict of
body is
true and IsSimpleParameterList of
parameters is
false, throw a
SyntaxError exception.
If any element of the BoundNames of
parameters also occurs in the LexicallyDeclaredNames of
body, throw a
SyntaxError exception.
If
body Contains
SuperCall is
true, throw a
SyntaxError exception.
If
parameters Contains
SuperCall is
true, throw a
SyntaxError exception.
If
body Contains
SuperProperty is
true, throw a
SyntaxError exception.
If
parameters Contains
SuperProperty is
true, throw a
SyntaxError exception.
If
kind is
"generator", then
If
parameters Contains
YieldExpression is
true, throw a
SyntaxError exception.
If
strict is
true, then
If BoundNames of
parameters contains any duplicate elements, throw a
SyntaxError exception.
A
prototype property is automatically created for every function created using CreateDynamicFunction, to provide
for the possibility that the function will be used as a constructor.
The Function constructor is itself a built-in function object. The value of the [[Prototype]] internal slot of the Function
constructor is the intrinsic object
%FunctionPrototype%.
The value of the [[Extensible]] internal slot of the Function constructor is
true.
The Function constructor has the following properties:
The value of
Function.prototype is
%FunctionPrototype%, the intrinsic Function prototype object.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
19.2.3Properties of the Function Prototype Object
#
The Function prototype object is the intrinsic object
%FunctionPrototype%. The Function prototype object is itself a built-in function object. When invoked, it accepts
any arguments and returns
undefined. It does not have a [[Construct]] internal method so it is not a constructor.
Note
The Function prototype object is specified to be a function object to ensure compatibility with ECMAScript code that
was created prior to the ECMAScript 2015 specification.
The value of the [[Prototype]] internal slot of the Function prototype object is the intrinsic object
%ObjectPrototype%. The initial value of the [[Extensible]] internal slot of the Function prototype object is
true.
The Function prototype object does not have a
prototype property.
The value of the
length property of the Function prototype object is 0.
The value of the
name property of the Function prototype object is the empty String.
The
thisArg value is passed without modification as the
this value. This is a change from Edition 3, where an
undefined or
nullthisArg is replaced with the
global object and
ToObject is applied to all other values and that result is passed as the
this value. Even though the
thisArg is passed without modification, non-strict functions still perform these transformations upon entry
to the function.
Note 2
If
func is an arrow function or a
bound function then the
thisArg will be ignored by the function [[Call]] in step 5.
If this method was called with more than one argument, then in left to right order, starting with the second argument,
append each argument as the last element of
argList.
The
thisArg value is passed without modification as the
this value. This is a change from Edition 3, where an
undefined or
nullthisArg is replaced with the
global object and
ToObject is applied to all other values and that result is passed as the
this value. Even though the
thisArg is passed without modification, non-strict functions still perform these transformations upon entry
to the function.
Note 2
If
func is an arrow function or a
bound function then the
thisArg will be ignored by the function [[Call]] in step 5.
When the
toString method is called on an object
func, the following steps are taken:
If
func is a Bound Function exotic object, then
Return an implementation-dependent String source code representation of
func. The representation must conform to the rules below. It is implementation dependent whether the representation
includes
bound function information or information about the target function.
If
Type(
func) is Object and is either a built-in function object or has an [[ECMAScriptCode]] internal slot, then
Return an implementation-dependent String source code representation of
func. The representation must conform to the rules below.
The use and placement of white space, line terminators, and semicolons within the representation String is implementation-dependent.
If the object was defined using ECMAScript code and the returned string representation is not in the form of a
MethodDefinition or
GeneratorMethod then the representation must be such that if the string is evaluated, using
eval in a lexical context that is equivalent to the lexical context used to create the original object, it
will result in a new functionally equivalent object. In that case the returned source code must not mention freely
any variables that were not mentioned freely by the original function's source code, even if these “extra” names were
originally in scope.
If the implementation cannot produce a source code string that meets these criteria then it must return a string for which
eval will throw a
SyntaxError exception.
19.2.3.6Function.prototype [ @@hasInstance ] (
V )
#
When the @@hasInstance method of an object
F is called with value
V, the following steps are taken:
The value of the
name property of this function is
"[Symbol.hasInstance]".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
Note
This is the default implementation of
@@hasInstance that most functions inherit.
@@hasInstance is called by the
instanceof operator to determine whether a value is an instance of a specific constructor. An expression such
as
v instanceof F
evaluates as
F[@@hasInstance](v)
A constructor function can control which objects are recognized as its instances by
instanceof by exposing a different
@@hasInstance method on the function.
This property is non-writable and non-configurable to prevent tampering that could be used to globally expose the target
function of a
bound function.
Every function instance is an ECMAScript function object and has the internal slots listed in
Table 27. Function instances created using the
Function.prototype.bind method (
19.2.3.2) have the internal slots listed in
Table 28.
The Function instances have the following properties:
The value of the
length property is an integer that indicates the typical number of arguments expected by the function. However,
the language permits the function to be invoked with some other number of arguments. The behaviour of a function when
invoked on a number of arguments other than the number specified by its
length property depends on the function. This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
The value of the
name property is an String that is descriptive of the function. The name has no semantic significance but is
typically a variable or property name that is used to refer to the function at its point of definition in ECMAScript
code. This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
Anonymous functions objects that do not have a contextual name associated with them by this specification do not have
a
name own property but inherit the
name property of
%FunctionPrototype%.
Function instances that can be used as a constructor have a
prototype property. Whenever such a function instance is created another ordinary object is also created and
is the initial value of the function's
prototype property. Unless otherwise specified, the value of the
prototype property is used to initialize the [[Prototype]] internal slot of the object created when that function
is invoked as a constructor.
This property has the attributes { [[Writable]]:
true, [[Enumerable]]:
false, [[Configurable]]:
false }.
Note
Function objects created using
Function.prototype.bind, or by evaluating a
MethodDefinition (that are not a
GeneratorMethod) or an
ArrowFunction grammar production do not have a
prototype property.
The Boolean constructor is the
%Boolean% intrinsic object and the initial value of the
Boolean property of the
global object. When called as a constructor it creates and initializes a new Boolean object. When
Boolean is called as a function rather than as a constructor, it performs a type conversion.
The
Boolean constructor is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass constructors that intend to inherit the specified
Boolean behaviour must include a
super call to the
Boolean constructor to create and initialize the subclass instance with a [[BooleanData]] internal slot.
The initial value of
Boolean.prototype is the intrinsic object
%BooleanPrototype%.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
19.3.3Properties of the Boolean Prototype Object
#
The Boolean prototype object is the intrinsic object
%BooleanPrototype%. The Boolean prototype object is an ordinary object. The Boolean prototype is itself a Boolean
object; it has a [[BooleanData]] internal slot with the value
false.
The value of the [[Prototype]] internal slot of the Boolean prototype object is the intrinsic object
%ObjectPrototype%.
Boolean instances are ordinary objects that inherit properties from the Boolean prototype object. Boolean instances have
a [[BooleanData]] internal slot. The [[BooleanData]] internal slot is the Boolean value represented by this Boolean
object.
The Symbol constructor is the
%Symbol% intrinsic object and the initial value of the
Symbol property of the
global object. When
Symbol is called as a function, it returns a new Symbol value.
The
Symbol constructor is not intended to be used with the
new operator or to be subclassed. It may be used as the value of an
extends clause of a class definition but a
super call to the
Symbol constructor will cause an exception.
For each element
e of the GlobalSymbolRegistry
List,
If
SameValue(
e.[[Key]],
stringKey) is
true, return
e.[[Symbol]].
Assert: GlobalSymbolRegistry does not currently contain an entry for
stringKey.
Let
newSymbol be a new unique Symbol value whose [[Description]] value is
stringKey.
Append the
Record { [[Key]]:
stringKey, [[Symbol]]:
newSymbol } to the GlobalSymbolRegistry
List.
Return
newSymbol.
The GlobalSymbolRegistry is a
List that is globally available. It is shared by all realms. Prior to the evaluation of any ECMAScript code it is initialized
as a new empty
List. Elements of the GlobalSymbolRegistry are Records with the structure defined in
Table 45.
The Symbol prototype object is the intrinsic object
%SymbolPrototype%. The Symbol prototype object is an ordinary object. It is not a Symbol instance and does not
have a [[SymbolData]] internal slot.
The value of the [[Prototype]] internal slot of the Symbol prototype object is the intrinsic object
%ObjectPrototype%.
If
Type(
s) is not Object, throw a
TypeError exception.
If
s does not have a [[SymbolData]] internal slot, throw a
TypeError exception.
Return the value of
s's [[SymbolData]] internal slot.
19.4.3.4Symbol.prototype [ @@toPrimitive ] (
hint )
#
This function is called by ECMAScript language operators to convert a Symbol object to a primitive value. The allowed
values for
hint are
"default",
"number", and
"string".
When the
@@toPrimitive method is called with argument
hint, the following steps are taken:
Symbol instances are ordinary objects that inherit properties from the Symbol prototype object. Symbol instances have
a [[SymbolData]] internal slot. The [[SymbolData]] internal slot is the Symbol value represented by this Symbol object.
Instances of Error objects are thrown as exceptions when runtime errors occur. The Error objects may also serve as base
objects for user-defined exception classes.
The Error constructor is the
%Error% intrinsic object and the initial value of the
Error property of the
global object. When
Error is called as a function rather than as a constructor, it creates and initializes a new Error object. Thus
the function call
Error(…) is equivalent to the object creation expression
new Error(…) with the same arguments.
The
Error constructor is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass constructors that intend to inherit the specified
Error behaviour must include a
super call to the
Error constructor to create and initialize subclass instances with a [[ErrorData]] internal slot.
The Error prototype object is the intrinsic object
%ErrorPrototype%. The Error prototype object is an ordinary object. It is not an Error instance and does not have
an [[ErrorData]] internal slot.
The value of the [[Prototype]] internal slot of the Error prototype object is the intrinsic object
%ObjectPrototype%.
Error instances are ordinary objects that inherit properties from the Error prototype object and have an [[ErrorData]]
internal slot whose value is
undefined. The only specified uses of [[ErrorData]] is to identify Error and
NativeError instances as Error objects within
Object.prototype.toString.
A new instance of one of the
NativeError objects below is thrown when a runtime error is detected. All of these objects share the same structure,
as described in
19.5.6.
When an ECMAScript implementation detects a runtime error, it throws a new instance of one of the
NativeError objects defined in
19.5.5. Each of these objects has the structure described below, differing only in the name used as the constructor name instead
of
NativeError, in the
name property of the prototype object, and in the implementation-defined
message property of the prototype object.
For each error object, references to
NativeError in the definition should be replaced with the appropriate error object name from
19.5.5.
When a
NativeError constructor is called as a function rather than as a constructor, it creates and initializes a new
NativeError object. A call of the object as a function is equivalent to calling it as a constructor with the
same arguments. Thus the function call
NativeError(…) is equivalent to the object creation expression
new
NativeError(…) with the same arguments.
Each
NativeError constructor is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass constructors that intend to inherit the specified
NativeError behaviour must include a
super call to the
NativeError constructor to create and initialize subclass instances with a [[ErrorData]] internal slot.
The actual value of the string passed in step 2 is either
"%EvalErrorPrototype%",
"%RangeErrorPrototype%",
"%ReferenceErrorPrototype%",
"%SyntaxErrorPrototype%",
"%TypeErrorPrototype%", or
"%URIErrorPrototype%" corresponding to which
NativeError constructor is being defined.
19.5.6.2Properties of the
NativeError Constructors
#
The value of the [[Prototype]] internal slot of a
NativeError constructor is the intrinsic object
%Error%.
Each
NativeError constructor has a
name property whose value is the String value `"
NativeError"`.
Each
NativeError constructor has the following properties:
The initial value of
NativeError.prototype is a
NativeError prototype object (
19.5.6.3). Each
NativeError constructor has a distinct prototype object.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
19.5.6.3Properties of the
NativeError Prototype Objects
#
Each
NativeError prototype object is an ordinary object. It is not an Error instance and does not have an [[ErrorData]]
internal slot.
The value of the [[Prototype]] internal slot of each
NativeError prototype object is the intrinsic object
%ErrorPrototype%.
The initial value of the
constructor property of the prototype for a given
NativeError constructor is the corresponding intrinsic object %
NativeError% (
19.5.6.1).
The initial value of the
name property of the prototype for a given
NativeError constructor is a string consisting of the name of the constructor (the name used instead of
NativeError).
NativeError instances are ordinary objects that inherit properties from their
NativeError prototype object and have an [[ErrorData]] internal slot whose value is
undefined. The only specified use of [[ErrorData]] is by
Object.prototype.toString (
19.1.3.6) to identify Error or
NativeError instances.
The Number constructor is the
%Number% intrinsic object and the initial value of the
Number property of the
global object. When called as a constructor, it creates and initializes a new Number object. When
Number is called as a function rather than as a constructor, it performs a type conversion.
The
Number constructor is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass constructors that intend to inherit the specified
Number behaviour must include a
super call to the
Number constructor to create and initialize the subclass instance with a [[NumberData]] internal slot.
The value of Number.EPSILON is the difference between 1 and the smallest value greater than 1 that is representable
as a Number value, which is approximately 2.2204460492503130808472633361816 x 10
-16.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
This function differs from the global isNaN function (
18.2.3) in that it does not convert its argument to a Number before determining whether it is
NaN.
The value of
Number.MIN_VALUE is the smallest positive value of the Number type, which is approximately
5 × 10
-324.
In the IEEE 754-2008 double precision binary representation, the smallest possible value is a denormalized number. If
an implementation does not support denormalized values, the value of
Number.MIN_VALUE must be the smallest non-zero positive value that can actually be represented by the implementation.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
The value of the
Number.parseFloat data property is the same built-in function object that is the value of the
parseFloat property of the
global object defined in
18.2.4.
The value of the
Number.parseInt data property is the same built-in function object that is the value of the
parseInt property of the
global object defined in
18.2.5.
The Number prototype object is the intrinsic object
%NumberPrototype%. The Number prototype object is an ordinary object. The Number prototype is itself a Number
object; it has a [[NumberData]] internal slot with the value
+0.
The value of the [[Prototype]] internal slot of the Number prototype object is the intrinsic object
%ObjectPrototype%.
Unless explicitly stated otherwise, the methods of the Number prototype object defined below are not generic and the
this value passed to them must be either a Number value or an object that has a [[NumberData]] internal slot
that has been initialized to a Number value.
The abstract operation thisNumberValue(
value) performs the following steps:
If
Type(
value) is Object and
value has a [[NumberData]] internal slot, then
Assert:
value's [[NumberData]] internal slot is a Number value.
Return the value of
value's [[NumberData]] internal slot.
Throw a
TypeError exception.
The phrase “this Number value” within the specification of a method refers to the result returned by calling the abstract
operation thisNumberValue with the
this value of the method invocation passed as the argument.
Return a String containing this Number value represented in decimal exponential notation with one digit before the significand's
decimal point and
fractionDigits digits after the significand's decimal point. If
fractionDigits is
undefined, include as many significand digits as necessary to uniquely specify the Number (just like in
ToString except that in this case the Number is always output in exponential notation). Specifically, perform the following
steps:
Return the concatenation of the Strings
s and
"Infinity".
If
f < 0 or
f > 20, throw a
RangeError exception. However, an implementation is permitted to extend the behaviour of
toExponential for values of
f less than 0 or greater than 20. In this case
toExponential would not necessarily throw
RangeError for such values.
If
x = 0, then
Let
m be the String consisting of
f+1 occurrences of the code unit 0x0030 (DIGIT ZERO).
Let
e be 0.
Else
x ≠ 0,
If
fractionDigits is not
undefined, then
Let
e and
n be integers such that 10
f ≤
n < 10
f+1 and for which the exact mathematical value of
n × 10
e-
f -
x is as close to zero as possible. If there are two such sets of
e and
n, pick the
e and
n for which
n × 10
e-
f is larger.
Else
fractionDigits is
undefined,
Let
e,
n, and
f be integers such that
f ≥ 0, 10
f ≤
n < 10
f+1, the Number value for
n × 10
e-
f is
x, and
f is as small as possible. Note that the decimal representation of
n has
f+1 digits,
n is not divisible by 10, and the least significant digit of
n is not necessarily uniquely determined by these criteria.
Let
m be the String consisting of the digits of the decimal representation of
n (in order, with no leading zeroes).
If
f ≠ 0, then
Let
a be the first element of
m, and let
b be the remaining
f elements of
m.
Let
m be the concatenation of the three Strings
a,
".", and
b.
If
e = 0, then
Let
c be
"+".
Let
d be
"0".
Else,
If
e > 0, let
c be
"+".
Else
e ≤ 0,
Let
c be
"-".
Let
e be -
e.
Let
d be the String consisting of the digits of the decimal representation of
e (in order, with no leading zeroes).
Let
m be the concatenation of the four Strings
m,
"e",
c, and
d.
Return the concatenation of the Strings
s and
m.
If the
toExponential method is called with more than one argument, then the behaviour is undefined (see clause
17).
Note
For implementations that provide more accurate conversions than required by the rules above, it is recommended that
the following alternative version of step 10.b.i be used as a guideline:
Let
e,
n, and
f be integers such that
f ≥ 0, 10
f ≤
n < 10
f+1, the Number value for
n × 10
e-
f is
x, and
f is as small as possible. If there are multiple possibilities for
n, choose the value of
n for which
n × 10
e-
f is closest in value to
x. If there are two such possible values of
n, choose the one that is even.
toFixed returns a String containing this Number value represented in decimal fixed-point notation with
fractionDigits digits after the decimal point. If
fractionDigits is
undefined, 0 is assumed.
The following steps are performed:
Let
x be ? thisNumberValue(
this value).
Let
f be ?
ToInteger(
fractionDigits). (If
fractionDigits is
undefined, this step produces the value 0.)
If
f < 0 or
f > 20, throw a
RangeError exception. However, an implementation is permitted to extend the behaviour of
toFixed for values of
f less than 0 or greater than 20. In this case
toFixed would not necessarily throw
RangeError for such values.
Let
n be an integer for which the exact mathematical value of
n ÷ 10
f -
x is as close to zero as possible. If there are two such
n, pick the larger
n.
If
n = 0, let
m be the String
"0". Otherwise, let
m be the String consisting of the digits of the decimal representation of
n (in order, with no leading zeroes).
If
f ≠ 0, then
Let
k be the number of elements in
m.
If
k ≤
f, then
Let
z be the String consisting of
f+1-
k occurrences of the code unit 0x0030 (DIGIT ZERO).
Let
m be the concatenation of Strings
z and
m.
Let
k be
f + 1.
Let
a be the first
k-
f elements of
m, and let
b be the remaining
f elements of
m.
Let
m be the concatenation of the three Strings
a,
".", and
b.
Return the concatenation of the Strings
s and
m.
If the
toFixed method is called with more than one argument, then the behaviour is undefined (see clause
17).
Note 2
The output of
toFixed may be more precise than
toString for some values because toString only prints enough significant digits to distinguish the number from
adjacent number values. For example,
(1000000000000000128).toString() returns
"1000000000000000100", while
(1000000000000000128).toFixed(0) returns
"1000000000000000128".
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the
Number.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation
does not include the ECMA-402 API the following specification of the
toLocaleString method is used.
Produces a String value that represents this Number value formatted according to the conventions of the host environment's
current locale. This function is implementation-dependent, and it is permissible, but not encouraged, for it to return
the same thing as
toString.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that
do not include ECMA-402 support must not use those parameter positions for anything else.
Return a String containing this Number value represented either in decimal exponential notation with one digit before
the significand's decimal point and
precision-1 digits after the significand's decimal point or in decimal fixed notation with
precision significant digits. If
precision is
undefined, call
ToString instead. Specifically, perform the following steps:
Return the String that is the concatenation of
s and
"Infinity".
If
p < 1 or
p > 21, throw a
RangeError exception. However, an implementation is permitted to extend the behaviour of
toPrecision for values of
p less than 1 or greater than 21. In this case
toPrecision would not necessarily throw
RangeError for such values.
If
x = 0, then
Let
m be the String consisting of
p occurrences of the code unit 0x0030 (DIGIT ZERO).
Let
e be 0.
Else
x ≠ 0,
Let
e and
n be integers such that 10
p-1 ≤
n < 10
p and for which the exact mathematical value of
n × 10
e-
p+1 -
x is as close to zero as possible. If there are two such sets of
e and
n, pick the
e and
n for which
n × 10
e-
p+1 is larger.
Let
m be the String consisting of the digits of the decimal representation of
n (in order, with no leading zeroes).
If
e < -6 or
e ≥
p, then
Assert:
e ≠ 0.
Let
a be the first element of
m, and let
b be the remaining
p-1 elements of
m.
Let
m be the concatenation of
a,
".", and
b.
If
e > 0, then
Let
c be code unit 0x002B (PLUS SIGN).
Else
e < 0,
Let
c be code unit 0x002D (HYPHEN-MINUS).
Let
e be -
e.
Let
d be the String consisting of the digits of the decimal representation of
e (in order, with no leading zeroes).
Return the concatenation of
s,
m, code unit 0x0065 (LATIN SMALL LETTER E),
c, and
d.
If
e =
p-1, return the concatenation of the Strings
s and
m.
If
e ≥ 0, then
Let
m be the concatenation of the first
e+1 elements of
m, the code unit 0x002E (FULL STOP), and the remaining
p- (
e+1) elements of
m.
Else
e < 0,
Let
m be the String formed by the concatenation of code unit 0x0030 (DIGIT ZERO), code unit 0x002E (FULL STOP),
-(
e+1) occurrences of code unit 0x0030 (DIGIT ZERO), and the String
m.
Return the String that is the concatenation of
s and
m.
If the
toPrecision method is called with more than one argument, then the behaviour is undefined (see clause
17).
The optional
radix should be an integer value in the inclusive range 2 to 36. If
radix not present or is
undefined the Number 10 is used as the value of
radix.
The following steps are performed:
Let
x be ? thisNumberValue(
this value).
If
radix is not present, let
radixNumber be 10.
Else if
radix is
undefined, let
radixNumber be 10.
Return the String representation of this Number value using the radix specified by
radixNumber. Letters
a-
z are used for digits with values 10 through 35. The precise algorithm is implementation-dependent, however
the algorithm should be a generalization of that specified in
7.1.12.1.
The
toString function is not generic; it throws a
TypeError exception if its
this value is not a Number or a Number object. Therefore, it cannot be transferred to other kinds of objects
for use as a method.
Number instances are ordinary objects that inherit properties from the Number prototype object. Number instances also
have a [[NumberData]] internal slot. The [[NumberData]] internal slot is the Number value represented by this Number
object.
The Math object is the
%Math% intrinsic object and the initial value of the
Math property of the
global object. The Math object is a single ordinary object.
The value of the [[Prototype]] internal slot of the Math object is the intrinsic object
%ObjectPrototype%.
The Math object is not a function object. It does not have a [[Construct]] internal method; it is not possible to use
the Math object as a constructor with the
new operator. The Math object also does not have a [[Call]] internal method; it is not possible to invoke the
Math object as a function.
Note
In this specification, the phrase “the Number value for
x” has a technical meaning defined in
6.1.6.
Each of the following
Math object functions applies the
ToNumber abstract operation to each of its arguments (in left-to-right order if there is more than one). If
ToNumber returns an
abrupt completion, that
Completion Record is immediately returned. Otherwise, the function performs a computation on the resulting Number value(s). The value
returned by each function is a Number.
In the function descriptions below, the symbols
NaN,
-0,
+0,
-∞ and
+∞ refer to the Number values described in
6.1.6.
Note
The behaviour of the functions
acos,
acosh,
asin,
asinh,
atan,
atanh,
atan2,
cbrt,
cos,
cosh,
exp,
expm1,
hypot,
log,
log1p,
log2,
log10,
pow,
random,
sin,
sinh,
sqrt,
tan, and
tanh is not precisely specified here except to require specific results for certain argument values that represent
boundary cases of interest. For other argument values, these functions are intended to compute approximations to the
results of familiar mathematical functions, but some latitude is allowed in the choice of approximation algorithms.
The general intent is that an implementer should be able to use the same mathematical library for ECMAScript on a given
hardware platform that is available to C programmers on that platform.
Although the choice of algorithms is left to the implementation, it is recommended (but not specified by this standard)
that implementations use the approximation algorithms for IEEE 754-2008 arithmetic contained in
fdlibm, the freely distributable mathematical library from Sun Microsystems (
http://www.netlib.org/fdlibm).
Returns an implementation-dependent approximation to the arc tangent of the quotient
y/
x of the arguments
y and
x, where the signs of
y and
x are used to determine the quadrant of the result. Note that it is intentional and traditional for the two-argument
arc tangent function that the argument named
y be first and the argument named
x be second. The result is expressed in radians and ranges from -π to +π.
If either
x or
y is
NaN, the result is
NaN.
If
y>0 and
x is
+0, the result is an implementation-dependent approximation to +π/2.
If
y>0 and
x is
-0, the result is an implementation-dependent approximation to +π/2.
If
y is
+0 and
x>0, the result is
+0.
If
y is
+0 and
x is
+0, the result is
+0.
If
y is
+0 and
x is
-0, the result is an implementation-dependent approximation to +π.
If
y is
+0 and
x<0, the result is an implementation-dependent approximation to +π.
If
y is
-0 and
x>0, the result is
-0.
If
y is
-0 and
x is
+0, the result is
-0.
If
y is
-0 and
x is
-0, the result is an implementation-dependent approximation to -π.
If
y is
-0 and
x<0, the result is an implementation-dependent approximation to -π.
If
y<0 and
x is
+0, the result is an implementation-dependent approximation to -π/2.
If
y<0 and
x is
-0, the result is an implementation-dependent approximation to -π/2.
If
y>0 and
y is finite and
x is
+∞, the result is
+0.
If
y>0 and
y is finite and
x is
-∞, the result is an implementation-dependent approximation to +π.
If
y<0 and
y is finite and
x is
+∞, the result is
-0.
If
y<0 and
y is finite and
x is
-∞, the result is an implementation-dependent approximation to -π.
If
y is
+∞ and
x is finite, the result is an implementation-dependent approximation to +π/2.
If
y is
-∞ and
x is finite, the result is an implementation-dependent approximation to -π/2.
If
y is
+∞ and
x is
+∞, the result is an implementation-dependent approximation to +π/4.
If
y is
+∞ and
x is
-∞, the result is an implementation-dependent approximation to +3π/4.
If
y is
-∞ and
x is
+∞, the result is an implementation-dependent approximation to -π/4.
If
y is
-∞ and
x is
-∞, the result is an implementation-dependent approximation to -3π/4.
Returns the smallest (closest to
-∞) Number value that is not less than
x and is equal to a mathematical integer. If
x is already an integer, the result is
x.
If
x is
NaN, the result is
NaN.
If
x is
+0, the result is
+0.
If
x is
-0, the result is
-0.
If
x is
+∞, the result is
+∞.
If
x is
-∞, the result is
-∞.
If
x is less than 0 but greater than -1, the result is
-0.
The value of
Math.ceil(x) is the same as the value of
-Math.floor(-x).
Returns an implementation-dependent approximation to the exponential function of
x (
e raised to the power of
x, where
e is the base of the natural logarithms).
Returns an implementation-dependent approximation to subtracting 1 from the exponential function of
x (
e raised to the power of
x, where
e is the base of the natural logarithms). The result is computed in a way that is accurate even when the value
of x is close 0.
Returns the greatest (closest to
+∞) Number value that is not greater than
x and is equal to a mathematical integer. If
x is already an integer, the result is
x.
If
x is
NaN, the result is
NaN.
If
x is
+0, the result is
+0.
If
x is
-0, the result is
-0.
If
x is
+∞, the result is
+∞.
If
x is
-∞, the result is
-∞.
If
x is greater than 0 but less than 1, the result is
+0.
Note
The value of
Math.floor(x) is the same as the value of
-Math.ceil(-x).
Math.hypot returns an implementation-dependent approximation of the square root of the sum of squares of its
arguments.
If no arguments are passed, the result is
+0.
If any argument is
+∞, the result is
+∞.
If any argument is
-∞, the result is
+∞.
If no argument is
+∞ or
-∞, and any argument is
NaN, the result is
NaN.
If all arguments are either
+0 or
-0, the result is
+0.
Note
Implementations should take care to avoid the loss of precision from overflows and underflows that are prone to occur
in naive implementations when this function is called with two or more arguments.
Returns an implementation-dependent approximation to the natural logarithm of 1 +
x. The result is computed in a way that is accurate even when the value of x is close to zero.
Given zero or more arguments, calls
ToNumber on each of the arguments and returns the largest of the resulting values.
If no arguments are given, the result is
-∞.
If any value is
NaN, the result is
NaN.
The comparison of values to determine the largest value is done using the
Abstract Relational Comparison algorithm except that
+0 is considered to be larger than
-0.
Given zero or more arguments, calls
ToNumber on each of the arguments and returns the smallest of the resulting values.
If no arguments are given, the result is
+∞.
If any value is
NaN, the result is
NaN.
The comparison of values to determine the smallest value is done using the
Abstract Relational Comparison algorithm except that
+0 is considered to be larger than
-0.
Returns a Number value with positive sign, greater than or equal to 0 but less than 1, chosen randomly or pseudo randomly
with approximately uniform distribution over that range, using an implementation-dependent algorithm or strategy. This
function takes no arguments.
Each
Math.random function created for distinct realms must produce a distinct sequence of values from successive
calls.
Returns the Number value that is closest to
x and is equal to a mathematical integer. If two integer Number values are equally close to
x, then the result is the Number value that is closer to
+∞. If
x is already an integer, the result is
x.
If
x is
NaN, the result is
NaN.
If
x is
+0, the result is
+0.
If
x is
-0, the result is
-0.
If
x is
+∞, the result is
+∞.
If
x is
-∞, the result is
-∞.
If
x is greater than 0 but less than 0.5, the result is
+0.
If
x is less than 0 but greater than or equal to -0.5, the result is
-0.
Note 1
Math.round(3.5) returns 4, but
Math.round(-3.5) returns -3.
Note 2
The value of
Math.round(x) is not always the same as the value of
Math.floor(x+0.5). When
x is
-0 or is less than 0 but greater than or equal to -0.5,
Math.round(x) returns
-0, but
Math.floor(x+0.5) returns
+0.
Math.round(x) may also differ from the value of
Math.floor(x+0.5)because of internal rounding when computing
x+0.5.
20.3.1Overview of Date Objects and Definitions of Abstract Operations
#
The following functions are abstract operations that operate on time values (defined in
20.3.1.1). Note that, in every case, if any argument to one of these functions is
NaN, the result will be
NaN.
A Date object contains a Number indicating a particular instant in time to within a millisecond. Such a Number is called
a
time value. A time value may also be
NaN, indicating that the Date object does not represent a specific instant of time.
Time is measured in ECMAScript in milliseconds since 01 January, 1970 UTC. In time values leap seconds are ignored.
It is assumed that there are exactly 86,400,000 milliseconds per day. ECMAScript Number values can represent all integers
from -9,007,199,254,740,992 to 9,007,199,254,740,992; this range suffices to measure times to millisecond precision
for any instant that is within approximately 285,616 years, either forward or backward, from 01 January, 1970 UTC.
The actual range of times supported by ECMAScript Date objects is slightly smaller: exactly -100,000,000 days to 100,000,000
days measured relative to midnight at the beginning of 01 January, 1970 UTC. This gives a range of 8,640,000,000,000,000
milliseconds to either side of 01 January, 1970 UTC.
The exact moment of midnight at the beginning of 01 January, 1970 UTC is represented by the value
+0.
ECMAScript uses an extrapolated Gregorian system to map a day number to a year number and to determine the month and
date within that year. In this system, leap years are precisely those which are (divisible by 4) and ((not divisible
by 100) or (divisible by 400)). The number of days in year number
y is therefore defined by
All non-leap years have 365 days with the usual number of days per month and leap years have an extra day in February.
The day number of the first day of year
y is given by:
A date number is identified by an integer in the range 1 through 31, inclusive. The mapping DateFromTime(
t) from a
time valuet to a date number is defined by:
An implementation of ECMAScript is expected to determine the local time zone adjustment. The local time zone adjustment
is a value
LocalTZA measured in milliseconds which when added to UTC represents the local
standard time. Daylight saving time is
not reflected by LocalTZA.
Note
It is recommended that implementations use the time zone information of the IANA Time Zone Database
http://www.iana.org/time-zones/.
An implementation dependent algorithm using best available information on time zones to determine the local daylight
saving time adjustment DaylightSavingTA(
t), measured in milliseconds. An implementation of ECMAScript is expected to make its best effort to determine
the local daylight saving time adjustment.
Note
It is recommended that implementations use the time zone information of the IANA Time Zone Database
http://www.iana.org/time-zones/.
The abstract operation MakeTime calculates a number of milliseconds from its four arguments, which must be ECMAScript
Number values. This operator functions as follows:
If
hour is not finite or
min is not finite or
sec is not finite or
ms is not finite, return
NaN.
Let
t be
h*msPerHour+m*msPerMinute+s*msPerSecond+milli, performing the arithmetic according to IEEE 754-2008 rules (that is, as if using the ECMAScript operators
* and
+).
The abstract operation MakeDay calculates a number of days from its three arguments, which must be ECMAScript Number
values. This operator functions as follows:
If
year is not finite or
month is not finite or
date is not finite, return
NaN.
Find a value
t such that
YearFromTime(
t) is
ym and
MonthFromTime(
t) is
mn and
DateFromTime(
t) is 1; but if this is not possible (because some argument is out of range), return
NaN.
The abstract operation MakeDate calculates a number of milliseconds from its two arguments, which must be ECMAScript
Number values. This operator functions as follows:
If
day is not finite or
time is not finite, return
NaN.
The abstract operation TimeClip calculates a number of milliseconds from its argument, which must be an ECMAScript Number
value. This operator functions as follows:
The point of step 4 is that an implementation is permitted a choice of internal representations of time values, for
example as a 64-bit signed integer or as a 64-bit floating-point value. Depending on the implementation, this internal
representation may or may not distinguish
-0 and
+0.
ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO 8601 Extended Format.
The format is as follows:
YYYY-MM-DDTHH:mm:ss.sssZ
Where the fields are as follows:
YYYY
is the decimal digits of the year 0000 to 9999 in the Gregorian calendar.
-
"-" (hyphen) appears literally twice in the string.
MM
is the month of the year from 01 (January) to 12 (December).
DD
is the day of the month from 01 to 31.
T
"T" appears literally in the string, to indicate the beginning of the time element.
HH
is the number of complete hours that have passed since midnight as two decimal digits from 00 to 24.
:
":" (colon) appears literally twice in the string.
mm
is the number of complete minutes since the start of the hour as two decimal digits from 00 to 59.
ss
is the number of complete seconds since the start of the minute as two decimal digits from 00 to 59.
.
"." (dot) appears literally in the string.
sss
is the number of complete milliseconds since the start of the second as three decimal digits.
Z
is the time zone offset specified as
"Z" (for UTC) or either
"+" or
"-" followed by a time expression
HH:mm
This format includes date-only forms:
YYYY
YYYY-MM
YYYY-MM-DD
It also includes “date-time” forms that consist of one of the above date-only forms immediately followed by one of the
following time forms with an optional time zone offset appended:
THH:mm
THH:mm:ss
THH:mm:ss.sss
All numbers must be base 10. If the
MM or
DD fields are absent
"01" is used as the value. If the
HH,
mm, or
ss fields are absent
"00" is used as the value and the value of an absent
sss field is
"000". When the time zone offset is absent, date-only forms are interpreted as a UTC time and date-time forms
are interpreted as a local time.
Illegal values (out-of-bounds as well as syntax errors) in a format string means that the format string is not a valid
instance of this format.
Note 1
As every day both starts and ends with midnight, the two notations
00:00 and
24:00 are available to distinguish the two midnights that can be associated with one date. This means that
the following two notations refer to exactly the same point in time:
1995-02-04T24:00 and
1995-02-05T00:00
Note 2
There exists no international standard that specifies abbreviations for civil time zones like CET, EST, etc. and sometimes
the same abbreviation is even used for two very different time zones. For this reason, ISO 8601 and this format specifies
numeric representations of date and time.
ECMAScript requires the ability to specify 6 digit years (extended years); approximately 285,426 years, either forward
or backward, from 01 January, 1970 UTC. To represent years before 0 or after 9999, ISO 8601 permits the expansion
of the year representation, but only by prior agreement between the sender and the receiver. In the simplified ECMAScript
format such an expanded year representation shall have 2 extra year digits and is always prefixed with a + or - sign.
The year 0 is considered positive and hence prefixed with a + sign.
The Date constructor is the
%Date% intrinsic object and the initial value of the
Date property of the
global object. When called as a constructor it creates and initializes a new Date object. When
Date is called as a function rather than as a constructor, it returns a String representing the current time
(UTC).
The
Date constructor is a single function whose behaviour is overloaded based upon the number and types of its arguments.
The
Date constructor is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass constructors that intend to inherit the specified
Date behaviour must include a
super call to the
Date constructor to create and initialize the subclass instance with a [[DateValue]] internal slot.
The
length property of the
Date constructor function is 7.
Let
tv be the result of parsing
v as a date, in exactly the same manner as for the
parse method (
20.3.3.2). If the parse resulted in an
abrupt completion,
tv is the
Completion Record.
The
parse function applies the
ToString operator to its argument. If
ToString results in an
abrupt completion the
Completion Record is immediately returned. Otherwise,
parse interprets the resulting String as a date and time; it returns a Number, the UTC
time value corresponding to the date and time. The String may be interpreted as a local time, a UTC time, or a time in some other
time zone, depending on the contents of the String. The function first attempts to parse the format of the String according
to the rules (including extended years) called out in Date Time String Format (
20.3.1.16). If the String does not conform to that format the function may fall back to any implementation-specific heuristics
or implementation-specific date formats. Unrecognizable Strings or dates containing illegal element values in the format
String shall cause
Date.parse to return
NaN.
If
x is any Date object whose milliseconds amount is zero within a particular implementation of ECMAScript, then
all of the following expressions should produce the same numeric value in that implementation, if all the properties
referenced have their initial values:
is not required to produce the same Number value as the preceding three expressions and, in general, the value produced
by
Date.parse is implementation-dependent when given any String value that does not conform to the Date Time String
Format (
20.3.1.16) and that could not be produced in that implementation by the
toString or
toUTCString method.
When the
UTC function is called with fewer than two arguments, the behaviour is implementation-dependent. When the
UTC function is called with two to seven arguments, it computes the date from
year,
month and (optionally)
date,
hours,
minutes,
seconds and
ms. The following steps are taken:
The
UTC function differs from the
Date constructor in two ways: it returns a
time value as a Number, rather than creating a Date object, and it interprets the arguments in UTC rather than as local time.
The Date prototype object is the intrinsic object
%DatePrototype%. The Date prototype object is itself an ordinary object. It is not a Date instance and does not
have a [[DateValue]] internal slot.
The value of the [[Prototype]] internal slot of the Date prototype object is the intrinsic object
%ObjectPrototype%.
Unless explicitly defined otherwise, the methods of the Date prototype object defined below are not generic and the
this value passed to them must be an object that has a [[DateValue]] internal slot that has been initialized
to a
time value.
The abstract operation thisTimeValue(
value) performs the following steps:
If
Type(
value) is Object and
value has a [[DateValue]] internal slot, then
Return the value of
value's [[DateValue]] internal slot.
Throw a
TypeError exception.
In following descriptions of functions that are properties of the Date prototype object, the phrase “this Date object”
refers to the object that is the
this value for the invocation of the function. If the Type of the
this value is not Object, a
TypeError exception is thrown. The phrase “
this time value” within the specification of a method refers to the result returned by calling the abstract operation
thisTimeValue with the
this value of the method invocation passed as the argument.
Set the [[DateValue]] internal slot of this Date object to
u.
Return
u.
The
length property of the
setFullYear method is 3.
Note
If
month is not specified, this method behaves as if
month were specified with the value
getMonth(). If
date is not specified, it behaves as if
date were specified with the value
getDate().
20.3.4.22Date.prototype.setHours (
hour [ ,
min [ ,
sec [ ,
ms ] ] ] )
#
Set the [[DateValue]] internal slot of this Date object to
u.
Return
u.
The
length property of the
setHours method is 4.
Note
If
min is not specified, this method behaves as if
min were specified with the value
getMinutes(). If
sec is not specified, it behaves as if
sec were specified with the value
getSeconds(). If
ms is not specified, it behaves as if
ms were specified with the value
getMilliseconds().
Set the [[DateValue]] internal slot of this Date object to
u.
Return
u.
The
length property of the
setMinutes method is 3.
Note
If
sec is not specified, this method behaves as if
sec were specified with the value
getSeconds(). If
ms is not specified, this behaves as if
ms were specified with the value
getMilliseconds().
20.3.4.25Date.prototype.setMonth (
month [ ,
date ] )
#
Set the [[DateValue]] internal slot of this Date object to
v.
Return
v.
The
length property of the
setUTCFullYear method is 3.
Note
If
month is not specified, this method behaves as if
month were specified with the value
getUTCMonth(). If
date is not specified, it behaves as if
date were specified with the value
getUTCDate().
20.3.4.30Date.prototype.setUTCHours (
hour [ ,
min [ ,
sec [ ,
ms ] ] ] )
#
Set the [[DateValue]] internal slot of this Date object to
v.
Return
v.
The
length property of the
setUTCHours method is 4.
Note
If
min is not specified, this method behaves as if
min were specified with the value
getUTCMinutes(). If
sec is not specified, it behaves as if
sec were specified with the value
getUTCSeconds(). If
ms is not specified, it behaves as if
ms were specified with the value
getUTCMilliseconds().
20.3.4.31Date.prototype.setUTCMilliseconds (
ms )
#
Set the [[DateValue]] internal slot of this Date object to
v.
Return
v.
The
length property of the
setUTCMinutes method is 3.
Note
If
sec is not specified, this method behaves as if
sec were specified with the value
getUTCSeconds(). If
ms is not specified, it function behaves as if
ms were specified with the value return by
getUTCMilliseconds().
20.3.4.33Date.prototype.setUTCMonth (
month [ ,
date ] )
#
This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent
the “date” portion of the Date in the current time zone in a convenient, human-readable form.
This function returns a String value representing the instance in time corresponding to
this time value. The format of the String is the Date Time string format defined in
20.3.1.16. All fields are present in the String. The time zone is always UTC, denoted by the suffix Z. If
this time value is not a finite Number or if the year is not a value that can be represented in that format (if necessary using extended
year format), a
RangeError exception is thrown.
The
toJSON function is intentionally generic; it does not require that its
this value be a Date object. Therefore, it can be transferred to other kinds of objects for use as a method.
However, it does require that any such object have a
toISOString method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the
Date.prototype.toLocaleDateString method as specified in the ECMA-402 specification. If an ECMAScript implementation
does not include the ECMA-402 API the following specification of the
toLocaleDateString method is used.
This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent
the “date” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the
conventions of the host environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that
do not include ECMA-402 support must not use those parameter positions for anything else.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the
Date.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation
does not include the ECMA-402 API the following specification of the
toLocaleString method is used.
This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent
the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host
environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that
do not include ECMA-402 support must not use those parameter positions for anything else.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the
Date.prototype.toLocaleTimeString method as specified in the ECMA-402 specification. If an ECMAScript implementation
does not include the ECMA-402 API the following specification of the
toLocaleTimeString method is used.
This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent
the “time” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the
conventions of the host environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that
do not include ECMA-402 support must not use those parameter positions for anything else.
For any Date object
d whose milliseconds amount is zero, the result of
Date.parse(d.toString()) is equal to
d.valueOf(). See
20.3.3.2.
Note 2
The
toString function is intentionally generic; it does not require that its
this value be a Date object. Therefore, it can be transferred to other kinds of objects for use as a method.
Return an implementation-dependent String value that represents
tv as a date and time in the current time zone using a convenient, human-readable form.
This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent
the “time” portion of the Date in the current time zone in a convenient, human-readable form.
This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent
this time value in a convenient, human-readable form in UTC.
Note
The intent is to produce a String representation of a date that is more readable than the format specified in
20.3.1.16. It is not essential that the chosen format be unambiguous or easily machine parsable. If an implementation does
not have a preferred human-readable format it is recommended to use the format defined in
20.3.1.16 but with a space rather than a
"T" used to separate the date and time elements.
20.3.4.45Date.prototype [ @@toPrimitive ] (
hint )
#
This function is called by ECMAScript language operators to convert a Date object to a primitive value. The allowed
values for
hint are
"default",
"number", and
"string". Date objects, are unique among built-in ECMAScript object in that they treat
"default" as being equivalent to
"string", All other built-in ECMAScript objects treat
"default" as being equivalent to
"number".
When the
@@toPrimitive method is called with argument
hint, the following steps are taken:
Let
O be the
this value.
If
Type(
O) is not Object, throw a
TypeError exception.
If
hint is the String value
"string" or the String value
"default", then
Let
tryFirst be
"string".
Else if
hint is the String value
"number", then
Let
tryFirst be
"number".
Else, throw a
TypeError exception.
Return ? OrdinaryToPrimitive(
O,
tryFirst).
The value of the
name property of this function is
"[Symbol.toPrimitive]".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
Date instances are ordinary objects that inherit properties from the Date prototype object. Date instances also have
a [[DateValue]] internal slot. The [[DateValue]] internal slot is the
time value represented by this Date object.
The String constructor is the
%String% intrinsic object and the initial value of the
String property of the
global object. When called as a constructor it creates and initializes a new String object. When
String is called as a function rather than as a constructor, it performs a type conversion.
The
String constructor is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass constructors that intend to inherit the specified
String behaviour must include a
super call to the
String constructor to create and initialize the subclass instance with a [[StringData]] internal slot.
The
String.raw function may be called with a variable number of arguments. The first argument is
template and the remainder of the arguments form the
Listsubstitutions. The following steps are taken:
Let
substitutions be a
List consisting of all of the arguments passed to this function, starting with the second argument. If fewer than two
arguments were passed, the
List is empty.
Let
numberOfSubstitutions be the number of elements in
substitutions.
Append in order the code unit elements of
nextSeg to the end of
stringElements.
If
nextIndex + 1 =
literalSegments, then
Return the String value whose code units are, in order, the elements in the
ListstringElements. If
stringElements has no elements, the empty string is returned.
If
nextIndex <
numberOfSubstitutions, let
next be
substitutions[
nextIndex].
Append in order the code unit elements of
nextSub to the end of
stringElements.
Let
nextIndex be
nextIndex + 1.
Note
String.raw is intended for use as a tag function of a Tagged Template (
12.3.7). When called as such, the first argument will be a well formed template object and the rest parameter will contain
the substitution values.
The String prototype object is the intrinsic object
%StringPrototype%. The String prototype object is an ordinary object. The String prototype is itself a String
object; it has a [[StringData]] internal slot with the value
"".
The value of the [[Prototype]] internal slot of the String prototype object is the intrinsic object
%ObjectPrototype%.
Unless explicitly stated otherwise, the methods of the String prototype object defined below are not generic and the
this value passed to them must be either a String value or an object that has a [[StringData]] internal slot
that has been initialized to a String value.
The abstract operation thisStringValue(
value) performs the following steps:
If
Type(
value) is Object and
value has a [[StringData]] internal slot, then
Assert:
value's [[StringData]] internal slot is a String value.
Return the value of
value's [[StringData]] internal slot.
Throw a
TypeError exception.
The phrase “this String value” within the specification of a method refers to the result returned by calling the abstract
operation thisStringValue with the
this value of the method invocation passed as the argument.
Returns a single element String containing the code unit at index
pos in the String value resulting from converting this object to a String. If there is no element at that index,
the result is the empty String. The result is a String value, not a String object.
If
pos is a value of Number type that is an integer, then the result of
x.charAt(pos) is equal to the result of
x.substring(pos, pos+1).
When the
charAt method is called with one argument
pos, the following steps are taken:
If
position < 0 or
position ≥
size, return the empty String.
Return a String of length 1, containing one code unit from
S, namely the code unit at index
position.
Note 2
The
charAt function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
Returns a Number (a nonnegative integer less than 2
16) that is the code unit value of the string element at index
pos in the String resulting from converting this object to a String. If there is no element at that index, the
result is
NaN.
When the
charCodeAt method is called with one argument
pos, the following steps are taken:
Return a value of Number type, whose value is the code unit value of the element at index
position in the String
S.
Note 2
The
charCodeAt function is intentionally generic; it does not require that its
this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
Returns a nonnegative integer Number less than 1114112 (0x110000) that is the code point value of the UTF-16 encoded
code point (
6.1.4) starting at the string element at index
pos in the String resulting from converting this object to a String. If there is no element at that index, the
result is
undefined. If a valid UTF-16 surrogate pair does not begin at
pos, the result is the code unit at
pos.
When the
codePointAt method is called with one argument
pos, the following steps are taken:
The
codePointAt function is intentionally generic; it does not require that its
this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
When the
concat method is called it returns a String consisting of the code units of the
this object (converted to a String) followed by the code units of each of the arguments converted to a String.
The result is a String value, not a String object.
When the
concat method is called with zero or more arguments, the following steps are taken:
Let
R be the String value consisting of the code units of the previous value of
R followed by the code units of
nextString.
Return
R.
The
length property of the
concat method is 1.
Note 2
The
concat function is intentionally generic; it does not require that its
this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
Let
searchLength be the number of elements in
searchStr.
Let
start be
end -
searchLength.
If
start is less than 0, return
false.
If the sequence of elements of
S starting at
start of length
searchLength is the same as the full element sequence of
searchStr, return
true.
Otherwise, return
false.
Note 1
Returns
true if the sequence of elements of
searchString converted to a String is the same as the corresponding elements of this object (converted to a
String) starting at
endPosition - length(this). Otherwise returns
false.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions
that allow such argument values.
Note 3
The
endsWith function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.7String.prototype.includes (
searchString [ ,
position ] )
#
The
includes method takes two arguments,
searchString and
position, and performs the following steps:
Let
searchLen be the number of elements in
searchStr.
If there exists any integer
k not smaller than
start such that
k +
searchLen is not greater than
len, and for all nonnegative integers
j less than
searchLen, the code unit at index
k+
j of
S is the same as the code unit at index
j of
searchStr, return
true; but if there is no such integer
k, return
false.
Note 1
If
searchString appears as a substring of the result of converting this object to a String, at one or more indices
that are greater than or equal to
position, return
true; otherwise, returns
false. If
position is
undefined, 0 is assumed, so as to search all of the String.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions
that allow such argument values.
Note 3
The
includes function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.8String.prototype.indexOf (
searchString [ ,
position ] )
#
Note 1
If
searchString appears as a substring of the result of converting this object to a String, at one or more indices
that are greater than or equal to
position, then the smallest such index is returned; otherwise,
-1 is returned. If
position is
undefined, 0 is assumed, so as to search all of the String.
The
indexOf method takes two arguments,
searchString and
position, and performs the following steps:
Let
searchLen be the number of elements in
searchStr.
Return the smallest possible integer
k not smaller than
start such that
k+
searchLen is not greater than
len, and for all nonnegative integers
j less than
searchLen, the code unit at index
k+
j of
S is the same as the code unit at index
j of
searchStr; but if there is no such integer
k, return the value
-1.
Note 2
The
indexOf function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.9String.prototype.lastIndexOf (
searchString [ ,
position ] )
#
Note 1
If
searchString appears as a substring of the result of converting this object to a String at one or more indices
that are smaller than or equal to
position, then the greatest such index is returned; otherwise,
-1 is returned. If
position is
undefined, the length of the String value is assumed, so as to search all of the String.
The
lastIndexOf method takes two arguments,
searchString and
position, and performs the following steps:
Let
searchLen be the number of elements in
searchStr.
Return the largest possible nonnegative integer
k not larger than
start such that
k+
searchLen is not greater than
len, and for all nonnegative integers
j less than
searchLen, the code unit at index
k+
j of
S is the same as the code unit at index
j of
searchStr; but if there is no such integer
k, return the value
-1.
Note 2
The
lastIndexOf function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the
localeCompare method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include
the ECMA-402 API the following specification of the
localeCompare method is used.
When the
localeCompare method is called with argument
that, it returns a Number other than
NaN that represents the result of a locale-sensitive String comparison of the
this value (converted to a String) with
that (converted to a String). The two Strings are
S and
That. The two Strings are compared in an implementation-defined fashion. The result is intended to order String
values in the sort order specified by a host default locale, and will be negative, zero, or positive, depending on
whether
S comes before
That in the sort order, the Strings are equal, or
S comes after
That in the sort order, respectively.
Before performing the comparisons, the following steps are performed to prepare the Strings:
The meaning of the optional second and third parameters to this method are defined in the ECMA-402 specification; implementations
that do not include ECMA-402 support must not assign any other interpretation to those parameter positions.
The
localeCompare method, if considered as a function of two arguments
this and
that, is a consistent comparison function (as defined in
22.1.3.25) on the set of all Strings.
The actual return values are implementation-defined to permit implementers to encode additional information in the value,
but the function is required to define a total ordering on all Strings. This function must treat Strings that are canonically
equivalent according to the Unicode standard as identical and must return
0 when comparing Strings that are considered canonically equivalent.
Note 1
The
localeCompare method itself is not directly suitable as an argument to
Array.prototype.sort because the latter requires a function of two arguments.
Note 2
This function is intended to rely on whatever language-sensitive comparison functionality is available to the ECMAScript
environment from the host environment, and to compare according to the rules of the host environment's current locale.
However, regardless of the host provided comparison capabilities, this function must treat Strings that are canonically
equivalent according to the Unicode standard as identical. It is recommended that this function should not honour
Unicode compatibility equivalences or decompositions. For a definition and discussion of canonical equivalence see
the Unicode Standard, chapters 2 and 3, as well as Unicode Standard Annex #15, Unicode Normalization Forms (
http://www.unicode.org/reports/tr15/) and Unicode Technical Note #5, Canonical Equivalence in Applications (
http://www.unicode.org/notes/tn5/). Also see Unicode Technical Standard #10, Unicode Collation Algorithm (
http://www.unicode.org/reports/tr10/).
Note 3
The
localeCompare function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
The
match function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.12String.prototype.normalize ( [
form ] )
#
When the
normalize method is called with one argument
form, the following steps are taken:
The
normalize function is intentionally generic; it does not require that its
this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
Let
T be a String value that is made from
n copies of
S appended together. If
n is 0,
T is the empty String.
Return
T.
Note 1
This method creates a String consisting of the code units of the
this object (converted to String) repeated
count times.
Note 2
The
repeat function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
Search
string for the first occurrence of
searchString and let
pos be the index within
string of the first code unit of the matched substring and let
matched be
searchString. If no occurrences of
searchString were found, return
string.
If
functionalReplace is
true, then
Let
replValue be ?
Call(
replaceValue,
undefined, «
matched,
pos,
string »).
Let
replStr be
GetSubstitution(
matched,
string,
pos,
captures,
replaceValue).
Let
tailPos be
pos + the number of code units in
matched.
Let
newString be the String formed by concatenating the first
pos code units of
string,
replStr, and the trailing substring of
string starting at index
tailPos. If
pos is 0, the first element of the concatenation will be the empty String.
Return
newString.
Note
The
replace function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
Let
result be a String value derived from
replacement by copying code unit elements from
replacement to
result while performing replacements as specified in
Table 46. These
$ replacements are done left-to-right, and, once such a replacement is performed, the new replacement text
is not subject to further replacements.
Return
result.
Table 46: Replacement Text Symbol Substitutions
Code units
Unicode Characters
Replacement text
0x0024, 0x0024
$$
$
0x0024, 0x0026
$&
matched
0x0024, 0x0060
$`
If
position is 0, the replacement is the empty String. Otherwise the replacement is the substring of
str that starts at index 0 and whose last code unit is at index
position - 1.
0x0024, 0x0027
$'
If
tailPos ≥
stringLength, the replacement is the empty String. Otherwise the replacement is the substring of
str that starts at index
tailPos and continues to the end of
str.
0x0024, N
Where
0x0031 ≤ N ≤ 0x0039
$n where
n is one of
1 2 3 4 5 6 7 8 9 and
$n is not followed by a decimal digit
The
nth element of
captures, where
n is a single digit in the range 1 to 9. If
n≤
m and the
nth element of
captures is
undefined, use the empty String instead. If
n>
m, the result is implementation-defined.
0x0024, N, N
Where
0x0030 ≤ N ≤ 0x0039
$nn where
n is one of
0 1 2 3 4 5 6 7 8 9
The
nnth element of
captures, where
nn is a two-digit decimal number in the range 01 to 99. If
nn≤
m and the
nnth element of
captures is
undefined, use the empty String instead. If
nn is 00 or
nn>
m, the result is implementation-defined.
0x0024
$ in any context that does not match any of the above.
The
search function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
The
slice method takes two arguments,
start and
end, and returns a substring of the result of converting this object to a String, starting from index
start and running to, but not including, index
end (or through the end of the String if
end is
undefined). If
start is negative, it is treated as
sourceLength+
start where
sourceLength is the length of the String. If
end is negative, it is treated as
sourceLength+
end where
sourceLength is the length of the String. The result is a String value, not a String object. The following steps
are taken:
Return a String value containing
span consecutive elements from
S beginning with the element at index
from.
Note
The
slice function is intentionally generic; it does not require that its
this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
Returns an Array object into which substrings of the result of converting this object to a String have been stored.
The substrings are determined by searching from left to right for occurrences of
separator; these occurrences are not part of any substring in the returned array, but serve to divide up the
String value. The value of
separator may be a String of any length or it may be an object, such as an RegExp, that has a @@split method.
When the
split method is called, the following steps are taken:
The value of
separator may be an empty String. In this case,
separator does not match the empty substring at the beginning or end of the input String, nor does it match
the empty substring at the end of the previous separator match. If
separator is the empty String, the String is split up into individual code unit elements; the length of the
result array equals the length of the String, and each substring contains one code unit.
If the
this object is (or converts to) the empty String, the result depends on whether
separator can match the empty String. If it can, the result array contains no elements. Otherwise, the result
array contains one element, which is the empty String.
If
separator is
undefined, then the result array contains just one String, which is the
this value (converted to a String). If
limit is not
undefined, then the output array is truncated so that it contains no more than
limit elements.
Note 2
The
split function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.17.1Runtime Semantics: SplitMatch (
S,
q,
R )
#
The abstract operation SplitMatch takes three parameters, a String
S, an integer
q, and a String
R, and performs the following steps in order to return either
false or the end index of a match:
If there exists an integer
i between 0 (inclusive) and
r (exclusive) such that the code unit at index
q+
i of
S is different from the code unit at index
i of
R, return
false.
Return
q+
r.
21.1.3.18String.prototype.startsWith (
searchString [ ,
position ] )
#
Let
searchLength be the number of elements in
searchStr.
If
searchLength+
start is greater than
len, return
false.
If the sequence of elements of
S starting at
start of length
searchLength is the same as the full element sequence of
searchStr, return
true.
Otherwise, return
false.
Note 1
This method returns
true if the sequence of elements of
searchString converted to a String is the same as the corresponding elements of this object (converted to a
String) starting at index
position. Otherwise returns
false.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions
that allow such argument values.
Note 3
The
startsWith function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.19String.prototype.substring (
start,
end )
#
The
substring method takes two arguments,
start and
end, and returns a substring of the result of converting this object to a String, starting from index
start and running to, but not including, index
end of the String (or through the end of the String if
end is
undefined). The result is a String value, not a String object.
If either argument is
NaN or negative, it is replaced with zero; if either argument is larger than the length of the String, it
is replaced with the length of the String.
Return a String whose length is
to -
from, containing code units from
S, namely the code units with indices
from through
to - 1, in ascending order.
Note
The
substring function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the
toLocaleLowerCase method as specified in the ECMA-402 specification. If an ECMAScript implementation does not
include the ECMA-402 API the following specification of the
toLocaleLowerCase method is used.
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4.
This function works exactly the same as
toLowerCase except that its result is intended to yield the correct result for the host environment's current
locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish)
where the rules for that language conflict with the regular Unicode case mappings.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that
do not include ECMA-402 support must not use those parameter positions for anything else.
Note
The
toLocaleLowerCase function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the
toLocaleUpperCase method as specified in the ECMA-402 specification. If an ECMAScript implementation does not
include the ECMA-402 API the following specification of the
toLocaleUpperCase method is used.
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4.
This function works exactly the same as
toUpperCase except that its result is intended to yield the correct result for the host environment's current
locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish)
where the rules for that language conflict with the regular Unicode case mappings.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that
do not include ECMA-402 support must not use those parameter positions for anything else.
Note
The
toLocaleUpperCase function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
Let
cpList be a
List containing in order the code points as defined in
6.1.4 of
S, starting at the first element of
S.
For each code point
c in
cpList, if the Unicode Character Database provides a language insensitive lower case equivalent of
c, then replace
c in
cpList with that equivalent code point(s).
For each code point
c in
cpList, in order, append to
cuList the elements of the
UTF16Encoding of
c.
Let
L be a String whose elements are, in order, the elements of
cuList.
Return
L.
The result must be derived according to the locale-insensitive case mappings in the Unicode Character Database (this
explicitly includes not only the UnicodeData.txt file, but also all locale-insensitive mappings in the SpecialCasings.txt
file that accompanies it).
Note 1
The case mapping of some code points may produce multiple code points. In this case the result String may not be the
same length as the source String. Because both
toUpperCase and
toLowerCase have context-sensitive behaviour, the functions are not symmetrical. In other words,
s.toUpperCase().toLowerCase() is not necessarily equal to
s.toLowerCase().
Note 2
The
toLowerCase function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4.
This function behaves in exactly the same way as
String.prototype.toLowerCase, except that code points are mapped to their
uppercase equivalents as specified in the Unicode Character Database.
Note
The
toUpperCase function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
Let
T be a String value that is a copy of
S with both leading and trailing white space removed. The definition of white space is the union of
WhiteSpace and
LineTerminator. When determining whether a Unicode code point is in Unicode general category “Zs”, code unit sequences are interpreted
as UTF-16 encoded code point sequences as specified in
6.1.4.
Return
T.
Note
The
trim function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
When the @@iterator method is called it returns an Iterator object (
25.1.1.2) that iterates over the code points of a String value, returning each code point as a String value. The following
steps are taken:
String instances are String exotic objects and have the internal methods specified for such objects. String instances
inherit properties from the String prototype object. String instances also have a [[StringData]] internal slot.
String instances have a
length property, and a set of enumerable properties with integer indexed names.
The number of elements in the String value represented by this String object.
Once a String object is initialized, this property is unchanging. It has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
An String Iterator is an object, that represents a specific iteration over some specific String instance object. There
is not a named constructor for String Iterator objects. Instead, String iterator objects are created by calling certain
methods of String instance objects.
Several methods of String objects return Iterator objects. The abstract operation CreateStringIterator with argument
string is used to create such iterator objects. It performs the following steps:
All String Iterator Objects inherit properties from the
%StringIteratorPrototype% intrinsic object. The %StringIteratorPrototype% object is an ordinary object and its
[[Prototype]] internal slot is the
%IteratorPrototype% intrinsic object. In addition, %StringIteratorPrototype% has the following properties:
String Iterator instances are ordinary objects that inherit properties from the
%StringIteratorPrototype% intrinsic object. String Iterator instances are initially created with the internal slots listed in
Table 47.
Table 47: Internal Slots of String Iterator Instances
Internal Slot
Description
[[IteratedString]]
The String value whose elements are being iterated.
[[StringIteratorNextIndex]]
The integer index of the next string index to be examined by this iteration.
The
RegExp constructor applies the following grammar to the input pattern String. An error occurs if the grammar
cannot interpret the String as an expansion of
Pattern.
A regular expression pattern is converted into an internal procedure using the process described below. An implementation
is encouraged to use more efficient algorithms than the ones listed below, as long as the results are the same. The
internal procedure is used as the value of a RegExp object's [[RegExpMatcher]] internal slot.
A
Pattern is either a BMP pattern or a Unicode pattern depending upon whether or not its associated flags contain a
"u". A BMP pattern matches against a String interpreted as consisting of a sequence of 16-bit values that are
Unicode code points in the range of the Basic Multilingual Plane. A Unicode pattern matches against a String interpreted
as consisting of Unicode code points encoded using UTF-16. In the context of describing the behaviour of a BMP pattern
“character” means a single 16-bit Unicode BMP code point. In the context of describing the behaviour of a Unicode pattern
“character” means a UTF-16 encoded code point (
6.1.4). In either context, “character value” means the numeric value of the corresponding non-encoded code point.
The syntax and semantics of
Pattern is defined as if the source code for the
Pattern was a
List of
SourceCharacter values where each
SourceCharacter corresponds to a Unicode code point. If a BMP pattern contains a non-BMP
SourceCharacter the entire pattern is encoded using UTF-16 and the individual code units of that encoding are used as the elements of
the
List.
Note
For example, consider a pattern expressed in source text as the single non-BMP character U+1D11E (MUSICAL SYMBOL G CLEF).
Interpreted as a Unicode pattern, it would be a single element (character)
List consisting of the single code point 0x1D11E. However, interpreted as a BMP pattern, it is first UTF-16 encoded to produce
a two element
List consisting of the code units 0xD834 and 0xDD1E.
Patterns are passed to the RegExp constructor as ECMAScript String values in which non-BMP characters are UTF-16 encoded.
For example, the single character MUSICAL SYMBOL G CLEF pattern, expressed as a String value, is a String of length
2 whose elements were the code units 0xD834 and 0xDD1E. So no further translation of the string would be necessary
to process it as a BMP pattern consisting of two pattern characters. However, to process it as a Unicode pattern
UTF16Decode must be used in producing a
List consisting of a single pattern character, the code point U+1D11E.
An implementation may not actually perform such translations to or from UTF-16, but the semantics of this specification
requires that the result of pattern matching be as if such translations were performed.
The descriptions below use the following variables:
Input is a
List consisting of all of the characters, in order, of the String being matched by the regular expression pattern. Each
character is either a code unit or a code point, depending upon the kind of pattern involved. The notation
Input[
n] means the
nth character of
Input, where
n can range between 0 (inclusive) and
InputLength (exclusive).
InputLength is the number of characters in
Input.
NcapturingParens is the total number of left capturing parentheses (i.e. the total number of times the
Atom::(Disjunction) production is expanded) in the pattern. A left capturing parenthesis is any
( pattern character that is matched by the
( terminal of the
Atom::(Disjunction) production.
IgnoreCase is
true if the RegExp object's [[OriginalFlags]] internal slot contains
"i" and otherwise is
false.
Multiline is
true if the RegExp object's [[OriginalFlags]] internal slot contains
"m" and otherwise is
false.
Unicode is
true if the RegExp object's [[OriginalFlags]] internal slot contains
"u" and otherwise is
false.
Furthermore, the descriptions below use the following internal data structures:
A
CharSet is a mathematical set of characters, either code units or code points depending up the state of the
Unicode flag. “All characters” means either all code unit values or all code point values also depending upon
the state if
Unicode.
A
State is an ordered pair (
endIndex,
captures) where
endIndex is an integer and
captures is a
List of
NcapturingParens values. States are used to represent partial match states in the regular expression matching
algorithms. The
endIndex is one plus the index of the last input character matched so far by the pattern, while
captures holds the results of capturing parentheses. The
nth element of
captures is either a
List that represents the value obtained by the
nth set of capturing parentheses or
undefined if the
nth set of capturing parentheses hasn't been reached yet. Due to backtracking, many States may be in use at any
time during the matching process.
A
MatchResult is either a State or the special token
failure that indicates that the match failed.
A
Continuation procedure is an internal closure (i.e. an internal procedure with some arguments already bound to
values) that takes one State argument and returns a MatchResult result. If an internal closure references variables
which are bound in the function that creates the closure, the closure uses the values that these variables had at
the time the closure was created. The Continuation attempts to match the remaining portion (specified by the closure's
already-bound arguments) of the pattern against
Input, starting at the intermediate state given by its State argument. If the match succeeds, the Continuation
returns the final State that it reached; if the match fails, the Continuation returns
failure.
A
Matcher procedure is an internal closure that takes two arguments — a State and a Continuation — and returns
a MatchResult result. A Matcher attempts to match a middle subpattern (specified by the closure's already-bound arguments)
of the pattern against
Input, starting at the intermediate state given by its State argument. The Continuation argument should be a
closure that matches the rest of the pattern. After matching the subpattern of a pattern to obtain a new State, the
Matcher then calls Continuation on that new State to test if the rest of the pattern can match as well. If it can,
the Matcher returns the State returned by Continuation; if not, the Matcher may try different choices at its choice
points, repeatedly calling Continuation until it either succeeds or all possibilities have been exhausted.
An
AssertionTester procedure is an internal closure that takes a State argument and returns a Boolean result. The
assertion tester tests a specific condition (specified by the closure's already-bound arguments) against the current
place in
Input and returns
true if the condition matched or
false if not.
An
EscapeValue is either a character or an integer. An EscapeValue is used to denote the interpretation of a
DecimalEscape escape sequence: a character
ch means that the escape sequence is interpreted as the character
ch, while an integer
n means that the escape sequence is interpreted as a backreference to the
nth set of capturing parentheses.
Return an internal closure that takes two arguments, a String
str and an integer
index, and performs the following steps:
Assert:
index ≤ the number of elements in
str.
If
Unicode is
true, let
Input be a
List consisting of the sequence of code points of
str interpreted as a UTF-16 encoded (
6.1.4) Unicode string. Otherwise, let
Input be a
List consisting of the sequence of code units that are the elements of
str.
Input will be used throughout the algorithms in
21.2.2. Each element of
Input is considered to be a character.
Let
InputLength be the number of characters contained in
Input. This variable will be used throughout the algorithms in
21.2.2.
Let
listIndex be the index into
Input of the character that was obtained from element
index of
str.
Let
c be a Continuation that always returns its State argument as a successful MatchResult.
Let
cap be a
List of
NcapturingParensundefined values, indexed 1 through
NcapturingParens.
Let
x be the State (
listIndex,
cap).
Call
m(
x,
c) and return its result.
Note
A Pattern evaluates (“compiles”) to an internal procedure value.
RegExpBuiltinExec can then apply this procedure to a String and an offset within the String to determine whether the pattern would match
starting at exactly that offset within the String, and, if it does match, what the values of the capturing parentheses
would be. The algorithms in
21.2.2 are designed so that compiling a pattern may throw a
SyntaxError exception; on the other hand, once the pattern is successfully compiled, applying the resulting
internal procedure to find a match in a String cannot throw an exception (except for any host-defined exceptions that
can occur anywhere such as out-of-memory).
Return an internal Matcher closure that takes two arguments, a State
x and a Continuation
c, and performs the following steps when evaluated:
Call
m1(
x,
c) and let
r be its result.
If
r is not
failure, return
r.
Call
m2(
x,
c) and return its result.
Note
The
| regular expression operator separates two alternatives. The pattern first tries to match the left
Alternative (followed by the sequel of the regular expression); if it fails, it tries to match the right
Disjunction (followed by the sequel of the regular expression). If the left
Alternative, the right
Disjunction, and the sequel all have choice points, all choices in the sequel are tried before moving on to the next choice in
the left
Alternative. If choices in the left
Alternative are exhausted, the right
Disjunction is tried instead of the left
Alternative. Any capturing parentheses inside a portion of the pattern skipped by
| produce
undefined values instead of Strings. Thus, for example,
The production
Alternative::[empty] evaluates by returning a Matcher that takes two arguments, a State
x and a Continuation
c, and returns the result of calling
c(
x).
Return an internal Matcher closure that takes two arguments, a State
x and a Continuation
c, and performs the following steps when evaluated:
Create a Continuation
d that takes a State argument
y and returns the result of calling
m2(
y,
c).
Call
m1(
x,
d) and return its result.
Note
Consecutive
Terms try to simultaneously match consecutive portions of
Input. If the left
Alternative, the right
Term, and the sequel of the regular expression all have choice points, all choices in the sequel are tried before moving
on to the next choice in the right
Term, and all choices in the right
Term are tried before moving on to the next choice in the left
Alternative.
The production
Term::Assertion evaluates by returning an internal Matcher closure that takes two arguments, a State
x and a Continuation
c, and performs the following steps when evaluated:
Evaluate
Assertion to obtain an AssertionTester
t.
Call
t(
x) and let
r be the resulting Boolean value.
Evaluate
Quantifier to obtain the three results: an integer
min, an integer (or ∞)
max, and Boolean
greedy.
If
max is finite and less than
min, throw a
SyntaxError exception.
Let
parenIndex be the number of left capturing parentheses in the entire regular expression that occur to the left
of this production expansion's
Term. This is the total number of times the
Atom::(Disjunction) production is expanded prior to this production's
Term plus the total number of
Atom::(Disjunction) productions enclosing this
Term.
Let
parenCount be the number of left capturing parentheses in the expansion of this production's
Atom. This is the total number of
Atom::(Disjunction) productions enclosed by this production's
Atom.
Return an internal Matcher closure that takes two arguments, a State
x and a Continuation
c, and performs the following steps when evaluated:
Call
RepeatMatcher(
m,
min,
max,
greedy,
x,
c,
parenIndex,
parenCount) and return its result.
The abstract operation RepeatMatcher takes eight parameters, a Matcher
m, an integer
min, an integer (or ∞)
max, a Boolean
greedy, a State
x, a Continuation
c, an integer
parenIndex, and an integer
parenCount, and performs the following steps:
If
max is zero, return
c(
x).
Create an internal Continuation closure
d that takes one State argument
y and performs the following steps when evaluated:
If
min is zero and
y's
endIndex is equal to
x's
endIndex, return
failure.
If
min is zero, let
min2 be zero; otherwise let
min2 be
min-1.
If
max is ∞, let
max2 be ∞; otherwise let
max2 be
max-1.
Call
RepeatMatcher(
m,
min2,
max2,
greedy,
y,
c,
parenIndex,
parenCount) and return its result.
For every integer
k that satisfies
parenIndex <
k and
k ≤
parenIndex+
parenCount, set
cap[
k] to
undefined.
Let
e be
x's
endIndex.
Let
xr be the State (
e,
cap).
If
min is not zero, return
m(
xr,
d).
If
greedy is
false, then
Call
c(
x) and let
z be its result.
If
z is not
failure, return
z.
Call
m(
xr,
d) and return its result.
Call
m(
xr,
d) and let
z be its result.
If
z is not
failure, return
z.
Call
c(
x) and return its result.
Note 1
An
Atom followed by a
Quantifier is repeated the number of times specified by the
Quantifier. A
Quantifier can be non-greedy, in which case the
Atom pattern is repeated as few times as possible while still matching the sequel, or it can be greedy, in which case
the
Atom pattern is repeated as many times as possible while still matching the sequel. The
Atom pattern is repeated rather than the input character sequence that it matches, so different repetitions of the
Atom can match different input substrings.
Note 2
If the
Atom and the sequel of the regular expression all have choice points, the
Atom is first matched as many (or as few, if non-greedy) times as possible. All choices in the sequel are tried before
moving on to the next choice in the last repetition of
Atom. All choices in the last (n
th) repetition of
Atom are tried before moving on to the next choice in the next-to-last (n-1)
st repetition of
Atom; at which point it may turn out that more or fewer repetitions of
Atom are now possible; these are exhausted (again, starting with either as few or as many as possible) before moving on
to the next choice in the (n-1)
st repetition of
Atom and so on.
Compare
/a[a-z]{2,4}/.exec("abcdefghi")
which returns
"abcde" with
/a[a-z]{2,4}?/.exec("abcdefghi")
which returns
"abc".
Consider also
/(aa|aabaac|ba|b|c)*/.exec("aabaac")
which, by the choice point ordering above, returns the array
["aaba", "ba"]
and not any of:
["aabaac", "aabaac"]
["aabaac", "c"]
The above ordering of choice points can be used to write a regular expression that calculates the greatest common
divisor of two numbers (represented in unary notation). The following example calculates the gcd of 10 and 15:
Step 4 of the RepeatMatcher clears
Atom's captures each time
Atom is repeated. We can see its behaviour in the regular expression
/(z)((a+)?(b+)?(c))*/.exec("zaacbbbcac")
which returns the array
["zaacbbbcac", "z", "ac", "a", undefined, "c"]
and not
["zaacbbbcac", "z", "ac", "a", "bbb", "c"]
because each iteration of the outermost
* clears all captured Strings contained in the quantified
Atom, which in this case includes capture Strings numbered 2, 3, 4, and 5.
Note 4
Step 1 of the RepeatMatcher's
d closure states that, once the minimum number of repetitions has been satisfied, any more expansions of
Atom that match the empty character sequence are not considered for further repetitions. This prevents the regular expression
engine from falling into an infinite loop on patterns such as:
The production
Assertion::^ evaluates by returning an internal AssertionTester closure that takes a State argument
x and performs the following steps when evaluated:
Let
e be
x's
endIndex.
If
e is zero, return
true.
If
Multiline is
false, return
false.
If the character
Input[
e-1] is one of
LineTerminator, return
true.
Return
false.
Note
Even when the
y flag is used with a pattern,
^ always matches only at the beginning of
Input, or (if
Multiline is
true) at the beginning of a line.
The production
Assertion::$ evaluates by returning an internal AssertionTester closure that takes a State argument
x and performs the following steps when evaluated:
Let
e be
x's
endIndex.
If
e is equal to
InputLength, return
true.
If
Multiline is
false, return
false.
If the character
Input[
e] is one of
LineTerminator, return
true.
Return
false.
The production
Assertion::\b evaluates by returning an internal AssertionTester closure that takes a State argument
x and performs the following steps when evaluated:
Let
e be
x's
endIndex.
Call
IsWordChar(
e-1) and let
a be the Boolean result.
Call
IsWordChar(
e) and let
b be the Boolean result.
If
a is
true and
b is
false, return
true.
If
a is
false and
b is
true, return
true.
Return
false.
The production
Assertion::\B evaluates by returning an internal AssertionTester closure that takes a State argument
x and performs the following steps when evaluated:
Let
e be
x's
endIndex.
Call
IsWordChar(
e-1) and let
a be the Boolean result.
Call
IsWordChar(
e) and let
b be the Boolean result.
Let
parenIndex be the number of left capturing parentheses in the entire regular expression that occur to the left
of this production expansion's initial left parenthesis. This is the total number of times the
Atom::(Disjunction) production is expanded prior to this production's
Atom plus the total number of
Atom::(Disjunction) productions enclosing this
Atom.
Return an internal Matcher closure that takes two arguments, a State
x and a Continuation
c, and performs the following steps:
Create an internal Continuation closure
d that takes one State argument
y and performs the following steps:
The abstract operation Canonicalize takes a character parameter
ch and performs the following steps:
If
IgnoreCase is
false, return
ch.
If
Unicode is
true, then
If the file CaseFolding.txt of the Unicode Character Database provides a simple or common case folding mapping
for
ch, return the result of applying that mapping to
ch.
Else, return
ch.
Else,
Assert:
ch is a UTF-16 code unit.
Let
s be the ECMAScript String value consisting of the single code unit
ch.
Let
u be the same result produced as if by performing the algorithm for
String.prototype.toUpperCase using
s as the
this value.
Assert:
u is a String value.
If
u does not consist of a single code unit, return
ch.
Let
cu be
u's single code unit element.
If
ch's code unit value ≥ 128 and
cu's code unit value < 128, return
ch.
Return
cu.
Note 1
Parentheses of the form
(Disjunction) serve both to group the components of the
Disjunction pattern together and to save the result of the match. The result can be used either in a backreference (
\ followed by a nonzero decimal number), referenced in a replace String, or returned as part of an array from
the regular expression matching internal procedure. To inhibit the capturing behaviour of parentheses, use the form
(?:Disjunction) instead.
Note 2
The form
(?=Disjunction) specifies a zero-width positive lookahead. In order for it to succeed, the pattern inside
Disjunction must match at the current position, but the current position is not advanced before matching the sequel. If
Disjunction can match at the current position in several ways, only the first one is tried. Unlike other regular expression operators,
there is no backtracking into a
(?= form (this unusual behaviour is inherited from Perl). This only matters when the
Disjunction contains capturing parentheses and the sequel of the pattern contains backreferences to those captures.
For example,
/(?=(a+))/.exec("baaabac")
matches the empty String immediately after the first
b and therefore returns the array:
["", "aaa"]
To illustrate the lack of backtracking into the lookahead, consider:
/(?=(a+))a*b\1/.exec("baaabac")
This expression returns
["aba", "a"]
and not:
["aaaba", "a"]
Note 3
The form
(?!Disjunction) specifies a zero-width negative lookahead. In order for it to succeed, the pattern inside
Disjunction must fail to match at the current position. The current position is not advanced before matching the sequel.
Disjunction can contain capturing parentheses, but backreferences to them only make sense from within
Disjunction itself. Backreferences to these capturing parentheses from elsewhere in the pattern always return
undefined because the negative lookahead must fail for the pattern to succeed. For example,
/(.*?)a(?!(a+)b\2c)\2(.*)/.exec("baaabaac")
looks for an
a not immediately followed by some positive number n of
a's, a
b, another n
a's (specified by the first
\2) and a
c. The second
\2 is outside the negative lookahead, so it matches against
undefined and therefore always succeeds. The whole expression returns the array:
["baaabaac", "ba", undefined, "abaac"]
Note 4
In case-insignificant matches when
Unicode is
true, all characters are implicitly case-folded using the simple mapping provided by the Unicode standard
immediately before they are compared. The simple mapping always maps to a single code point, so it does not map,
for example,
"ß" (U+00DF) to
"SS". It may however map a code point outside the Basic Latin range to a character within, for example,
"ſ" (U+017F) to
"s". Such characters are not mapped if
Unicode is
false. This prevents Unicode code points such as U+017F and U+212A from matching regular expressions such
as
/[a-z]/i, but they will match
/[a-z]/ui.
If there exists an integer
i between 0 (inclusive) and
len (exclusive) such that
Canonicalize(
s[
i]) is not the same character value as
Canonicalize(
Input[
e+
i]), return
failure.
An escape sequence of the form
\ followed by a nonzero decimal number
n matches the result of the
nth set of capturing parentheses (see 0). It is an error if the regular expression has fewer than
n capturing parentheses. If the regular expression has
n or more capturing parentheses but the
nth one is
undefined because it has not captured anything, then the backreference always succeeds.
If
\ is followed by a decimal number
n whose first digit is not
0, then the escape sequence is considered to be a backreference. It is an error if
n is greater than the total number of left capturing parentheses in the entire regular expression.
\0 represents the <NUL> character and cannot be followed by a decimal digit.
The production
CharacterClassEscape::s evaluates by returning the set of characters containing the characters that are on the right-hand side of the
WhiteSpace or
LineTerminator productions.
Call
CharacterRange(
A,
B) and let
D be the resulting CharSet.
Return the union of CharSets
D and
C.
Note 1
ClassRanges can expand into a single
ClassAtom and/or ranges of two
ClassAtom separated by dashes. In the latter case the
ClassRanges includes all characters between the first
ClassAtom and the second
ClassAtom, inclusive; an error occurs if either
ClassAtom does not represent a single character (for example, if one is \w) or if the first
ClassAtom's character value is greater than the second
ClassAtom's character value.
Note 2
Even if the pattern ignores case, the case of the two ends of a range is significant in determining which characters
belong to the range. Thus, for example, the pattern
/[E-F]/i matches only the letters
E,
F,
e, and
f, while the pattern
/[E-f]/i matches all upper and lower-case letters in the Unicode Basic Latin block as well as the symbols
[,
\,
],
^,
_, and
`.
Note 3
A
- character can be treated literally or it can denote a range. It is treated literally if it is the first or
last character of
ClassRanges, the beginning or end limit of a range specification, or immediately follows a range specification.
A
ClassAtom can use any of the escape sequences that are allowed in the rest of the regular expression except for
\b,
\B, and backreferences. Inside a
CharacterClass,
\b means the backspace character, while
\B and backreferences raise errors. Using a backreference inside a
ClassAtom causes an error.
The RegExp constructor is the
%RegExp% intrinsic object and the initial value of the
RegExp property of the
global object. When
RegExp is called as a function rather than as a constructor, it creates and initializes a new RegExp object.
Thus the function call
RegExp(…) is equivalent to the object creation expression
new RegExp(…) with the same arguments.
The
RegExp constructor is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass constructors that intend to inherit the specified
RegExp behaviour must include a
super call to the
RegExp constructor to create and initialize subclass instances with the necessary internal slots.
If pattern is supplied using a
StringLiteral, the usual escape sequence substitutions are performed before the String is processed by RegExp. If pattern must
contain an escape sequence to be recognized by RegExp, any U+005C (REVERSE SOLIDUS) code points must be escaped within
the
StringLiteral to prevent them being removed when the contents of the
StringLiteral are formed.
21.2.3.2Abstract Operations for the RegExp Constructor
#
If
F contains any code unit other than
"g",
"i",
"m",
"u", or
"y" or if it contains the same code unit more than once, throw a
SyntaxError exception.
If
F contains
"u", let
BMP be
false; else let
BMP be
true.
If
BMP is
true, then
Parse
P using the grammars in
21.2.1 and interpreting each of its 16-bit elements as a Unicode BMP code point. UTF-16 decoding is not applied to the
elements. The goal symbol for the parse is
Pattern. Throw a
SyntaxError exception if
P did not conform to the grammar, if any elements of
P were not matched by the parse, or if any Early Error conditions exist.
Let
patternCharacters be a
List whose elements are the code unit elements of
P.
Else,
Parse
P using the grammars in
21.2.1 and interpreting
P as UTF-16 encoded Unicode code points (
6.1.4). The goal symbol for the parse is
Pattern[U]. Throw a
SyntaxError exception if
P did not conform to the grammar, if any elements of
P were not matched by the parse, or if any Early Error conditions exist.
Let
patternCharacters be a
List whose elements are the code points resulting from applying UTF-16 decoding to
P's sequence of elements.
Set the value of
obj's [[OriginalSource]] internal slot to
P.
Set the value of
obj's [[OriginalFlags]] internal slot to
F.
Set
obj's [[RegExpMatcher]] internal slot to the internal procedure that evaluates the above parse of
P by applying the semantics provided in
21.2.2 using
patternCharacters as the pattern's
List of
SourceCharacter values and
F as the flag parameters.
21.2.3.2.4Runtime Semantics: EscapeRegExpPattern (
P,
F )
#
When the abstract operation EscapeRegExpPattern with arguments
P and
F is called, the following occurs:
Let
S be a String in the form of a
Pattern (
Pattern[U] if
F contains
"u") equivalent to
P interpreted as UTF-16 encoded Unicode code points (
6.1.4), in which certain code points are escaped as described below.
S may or may not be identical to
P; however, the internal procedure that would result from evaluating
S as a
Pattern (
Pattern[U] if
F contains
"u") must behave identically to the internal procedure given by the constructed object's [[RegExpMatcher]]
internal slot. Multiple calls to this abstract operation using the same values for
P and
F must produce identical results.
The code points
/ or any
LineTerminator occurring in the pattern shall be escaped in
S as necessary to ensure that the String value formed by concatenating the Strings
"/",
S,
"/", and
F can be parsed (in an appropriate lexical context) as a
RegularExpressionLiteral that behaves identically to the constructed regular expression. For example, if
P is
"/", then
S could be
"\/" or
"\u002F", among other possibilities, but not
"/", because
/// followed by
F would be parsed as a
SingleLineComment rather than a
RegularExpressionLiteral. If
P is the empty String, this specification can be met by letting
S be
"(?:)".
RegExp[@@species] is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps:
Return the
this value.
The value of the
name property of this function is
"get [Symbol.species]".
Note
RegExp prototype methods normally use their
this object's constructor to create a derived object. However, a subclass constructor may over-ride that default
behaviour by redefining its @@species property.
The RegExp prototype object is the intrinsic object
%RegExpPrototype%. The RegExp prototype object is an ordinary object. It is not a RegExp instance and does not
have a [[RegExpMatcher]] internal slot or any of the other internal slots of RegExp instance objects.
The value of the [[Prototype]] internal slot of the RegExp prototype object is the intrinsic object
%ObjectPrototype%.
Note
The RegExp prototype object does not have a
valueOf property of its own; however, it inherits the
valueOf property from the Object prototype object.
Performs a regular expression match of
string against the regular expression and returns an Array object containing the results of the match, or
null if
string did not match.
The String
ToString(
string) is searched for an occurrence of the regular expression pattern as follows:
Let
R be the
this value.
If
Type(
R) is not Object, throw a
TypeError exception.
If
R does not have a [[RegExpMatcher]] internal slot, throw a
TypeError exception.
If a callable
exec property is not found this algorithm falls back to attempting to use the built-in RegExp matching algorithm.
This provides compatible behaviour for code written for prior editions where most built-in algorithms that use regular
expressions did not perform a dynamic property lookup of
exec.
21.2.5.2.2Runtime Semantics: RegExpBuiltinExec (
R,
S )
#
The abstract operation RegExpBuiltinExec with arguments
R and
S performs the following steps:
e is an index into the
Input character list, derived from
S, matched by
matcher. Let
eUTF be the smallest index into
S that corresponds to the character at element
e of
Input. If
e is greater than or equal to the length of
Input, then
eUTF is the number of code units in
S.
The value of the
name property of this function is
"[Symbol.match]".
Note
The @@match property is used by the
IsRegExp abstract operation to identify objects that have the basic behaviour of regular expressions. The absence of a @@match
property or the existence of such a property whose value does not Boolean coerce to
true indicates that the object is not intended to be used as a regular expression object.
Let
replacement be
GetSubstitution(
matched,
S,
position,
captures,
replaceValue).
If
position ≥
nextSourcePosition, then
NOTE
position should not normally move backwards. If it does, it is an indication of an ill-behaving RegExp
subclass or use of an access triggered side-effect to change the global flag or other characteristics of
rx. In such cases, the corresponding substitution is ignored.
Let
accumulatedResult be the String formed by concatenating the code units of the current value of
accumulatedResult with the substring of
S consisting of the code units from
nextSourcePosition (inclusive) up to
position (exclusive) and with the code units of
replacement.
Let
nextSourcePosition be
position +
matchLength.
If
nextSourcePosition ≥
lengthS, return
accumulatedResult.
Return the String formed by concatenating the code units of
accumulatedResult with the substring of
S consisting of the code units from
nextSourcePosition (inclusive) up through the final code unit of
S (inclusive).
The value of the
name property of this function is
"[Symbol.replace]".
Returns an Array object into which substrings of the result of converting
string to a String have been stored. The substrings are determined by searching from left to right for matches
of the
this value regular expression; these occurrences are not part of any substring in the returned array, but
serve to divide up the String value.
The
this value may be an empty regular expression or a regular expression that can match an empty String. In
this case, the regular expression does not match the empty substring at the beginning or end of the input String,
nor does it match the empty substring at the end of the previous separator match. (For example, if the regular expression
matches the empty String, the String is split up into individual code unit elements; the length of the result array
equals the length of the String, and each substring contains one code unit.) Only the first match at a given index
of the String is considered, even if backtracking could yield a non-empty-substring match at that index. (For example,
/a*?/[Symbol.split]("ab") evaluates to the array
["a","b"], while
/a*/[Symbol.split]("ab") evaluates to the array
["","b"].)
If the
string is (or converts to) the empty String, the result depends on whether the regular expression can match
the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one element,
which is the empty String.
If the regular expression contains capturing parentheses, then each time
separator is matched the results (including any
undefined results) of the capturing parentheses are spliced into the output array. For example,
RegExp instances are ordinary objects that inherit properties from the RegExp prototype object. RegExp instances have
internal slots [[RegExpMatcher]], [[OriginalSource]], and [[OriginalFlags]]. The value of the [[RegExpMatcher]] internal
slot is an implementation dependent representation of the
Pattern of the RegExp object.
Note
Prior to ECMAScript 2015,
RegExp instances were specified as having the own data properties
source,
global,
ignoreCase, and
multiline. Those properties are now specified as accessor properties of RegExp.prototype.
RegExp instances also have the following property:
The value of the
lastIndex property specifies the String index at which to start the next match. It is coerced to an integer
when used (see
21.2.5.2.2). This property shall have the attributes { [[Writable]]:
true, [[Enumerable]]:
false, [[Configurable]]:
false }.
Array objects are exotic objects that give special treatment to a certain class of property names. See
9.4.2 for a definition of this special treatment.
The Array constructor is the
%Array% intrinsic object and the initial value of the
Array property of the
global object. When called as a constructor it creates and initializes a new exotic Array object. When
Array is called as a function rather than as a constructor, it also creates and initializes a new Array object.
Thus the function call
Array(…) is equivalent to the object creation expression
new Array(…) with the same arguments.
The
Array constructor is a single function whose behaviour is overloaded based upon the number and types of its arguments.
The
Array constructor is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass constructors that intend to inherit the exotic
Array behaviour must include a
super call to the
Array constructor to initialize subclass instances that are exotic Array objects. However, most of the
Array.prototype methods are generic methods that are not dependent upon their
this value being an exotic Array object.
The
length property of the
Array constructor function is 1.
The
from function is an intentionally generic factory method; it does not require that its
this value be the Array constructor. Therefore it can be transferred to or inherited by any other constructors
that may be called with a single numeric argument.
The
items argument is assumed to be a well-formed rest argument value.
Note 2
The
of function is an intentionally generic factory method; it does not require that its
this value be the Array constructor. Therefore it can be transferred to or inherited by other constructors
that may be called with a single numeric argument.
Array[@@species] is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps:
Return the
this value.
The value of the
name property of this function is
"get [Symbol.species]".
Note
Array prototype methods normally use their
this object's constructor to create a derived object. However, a subclass constructor may over-ride that default
behaviour by redefining its @@species property.
The Array prototype object is the intrinsic object
%ArrayPrototype%. The Array prototype object is an Array exotic objects and has the internal methods specified
for such objects. It has a
length property whose initial value is 0 and whose attributes are { [[Writable]]:
true, [[Enumerable]]:
false, [[Configurable]]:
false }.
The value of the [[Prototype]] internal slot of the Array prototype object is the intrinsic object
%ObjectPrototype%.
Note
The Array prototype object is specified to be an Array exotic object to ensure compatibility with ECMAScript code that
was created prior to the ECMAScript 2015 specification.
When the
concat method is called with zero or more arguments, it returns an array containing the array elements of the
object followed by the array elements of each argument in order.
Let
items be a
List whose first element is
O and whose subsequent elements are, in left to right order, the arguments that were passed to this function
invocation.
Repeat, while
items is not empty
Remove the first element from
items and let
E be the value of the element.
The explicit setting of the
length property in step 6 is necessary to ensure that its value is correct in situations where the trailing
elements of the result Array are not present.
Note 2
The
concat function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.1.1Runtime Semantics: IsConcatSpreadable (
O )
#
The abstract operation IsConcatSpreadable with argument
O performs the following steps:
The initial value of
Array.prototype.constructor is the intrinsic object
%Array%.
22.1.3.3Array.prototype.copyWithin (
target,
start [ ,
end ] )
#
The
copyWithin method takes up to three arguments
target,
start and
end.
Note 1
The
end argument is optional with the length of the
this object as its default value. If
target is negative, it is treated as
length+
target where
length is the length of the array. If
start is negative, it is treated as
length+
start. If
end is negative, it is treated as
length+
end.
The
copyWithin function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean
value
true or
false.
every calls
callbackfn once for each element present in the array, in ascending order, until it finds one where
callbackfn returns
false. If such an element is found,
every immediately returns
false. Otherwise, if
callbackfn returned
true for all elements,
every will return
true.
callbackfn is called only for elements of the array which actually exist; it is not called for missing elements
of the array.
If a
thisArg parameter is provided, it will be used as the
this value for each invocation of
callbackfn. If it is not provided,
undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object
being traversed.
every does not directly mutate the object on which it is called but the object may be mutated by the calls
to
callbackfn.
The range of elements processed by
every is set before the first call to
callbackfn. Elements which are appended to the array after the call to
every begins will not be visited by
callbackfn. If existing elements of the array are changed, their value as passed to
callbackfn will be the value at the time
every visits them; elements that are deleted after the call to
every begins and before being visited are not visited.
every acts like the "for all" quantifier in mathematics. In particular, for an empty array, it returns
true.
When the
every method is called with one or two arguments, the following steps are taken:
Let
testResult be
ToBoolean(?
Call(
callbackfn,
T, «
kValue,
k,
O »)).
If
testResult is
false, return
false.
Increase
k by 1.
Return
true.
Note 2
The
every function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.6Array.prototype.fill (
value [ ,
start [ ,
end ] ] )
#
The
fill method takes up to three arguments
value,
start and
end.
Note 1
The
start and
end arguments are optional with default values of 0 and the length of the
this object. If
start is negative, it is treated as
length+
start where
length is the length of the array. If
end is negative, it is treated as
length+
end.
The
fill function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean
value
true or
false.
filter calls
callbackfn once for each element in the array, in ascending order, and constructs a new array of all the values
for which
callbackfn returns
true.
callbackfn is called only for elements of the array which actually exist; it is not called for missing elements
of the array.
If a
thisArg parameter is provided, it will be used as the
this value for each invocation of
callbackfn. If it is not provided,
undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object
being traversed.
filter does not directly mutate the object on which it is called but the object may be mutated by the calls
to
callbackfn.
The range of elements processed by
filter is set before the first call to
callbackfn. Elements which are appended to the array after the call to
filter begins will not be visited by
callbackfn. If existing elements of the array are changed their value as passed to
callbackfn will be the value at the time
filter visits them; elements that are deleted after the call to
filter begins and before being visited are not visited.
When the
filter method is called with one or two arguments, the following steps are taken:
The
filter function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The
find method is called with one or two arguments,
predicate and
thisArg.
Note 1
predicate should be a function that accepts three arguments and returns a value that is coercible to a Boolean
value.
find calls
predicate once for each element of the array, in ascending order, until it finds one where
predicate returns
true. If such an element is found,
find immediately returns that element value. Otherwise,
find returns
undefined.
If a
thisArg parameter is provided, it will be used as the
this value for each invocation of
predicate. If it is not provided,
undefined is used instead.
predicate is called with three arguments: the value of the element, the index of the element, and the object
being traversed.
find does not directly mutate the object on which it is called but the object may be mutated by the calls to
predicate.
The range of elements processed by
find is set before the first call to
callbackfn. Elements that are appended to the array after the call to
find begins will not be visited by
callbackfn. If existing elements of the array are changed, their value as passed to
predicate will be the value at the time that
find visits them.
When the
find method is called, the following steps are taken:
Let
testResult be
ToBoolean(?
Call(
predicate,
T, «
kValue,
k,
O »)).
If
testResult is
true, return
kValue.
Increase
k by 1.
Return
undefined.
Note 2
The
find function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
predicate should be a function that accepts three arguments and returns a value that is coercible to the Boolean
value
true or
false.
findIndex calls
predicate once for each element of the array, in ascending order, until it finds one where
predicate returns
true. If such an element is found,
findIndex immediately returns the index of that element value. Otherwise,
findIndex returns -1.
If a
thisArg parameter is provided, it will be used as the
this value for each invocation of
predicate. If it is not provided,
undefined is used instead.
predicate is called with three arguments: the value of the element, the index of the element, and the object
being traversed.
findIndex does not directly mutate the object on which it is called but the object may be mutated by the calls
to
predicate.
The range of elements processed by
findIndex is set before the first call to
callbackfn. Elements that are appended to the array after the call to
findIndex begins will not be visited by
callbackfn. If existing elements of the array are changed, their value as passed to
predicate will be the value at the time that
findIndex visits them.
When the
findIndex method is called with one or two arguments, the following steps are taken:
Let
testResult be
ToBoolean(?
Call(
predicate,
T, «
kValue,
k,
O »)).
If
testResult is
true, return
k.
Increase
k by 1.
Return -1.
Note 2
The
findIndex function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments.
forEach calls
callbackfn once for each element present in the array, in ascending order.
callbackfn is called only for elements of the array which actually exist; it is not called for missing elements
of the array.
If a
thisArg parameter is provided, it will be used as the
this value for each invocation of
callbackfn. If it is not provided,
undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object
being traversed.
forEach does not directly mutate the object on which it is called but the object may be mutated by the calls
to
callbackfn.
When the
forEach method is called with one or two arguments, the following steps are taken:
The
forEach function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
includes compares
searchElement to the elements of the array, in ascending order, using the
SameValueZero algorithm, and if found at any position, returns
true; otherwise,
false is returned.
The optional second argument
fromIndex defaults to 0 (i.e. the whole array is searched). If it is greater than or equal to the length of
the array,
false is returned, i.e. the array will not be searched. If it is negative, it is used as the offset from
the end of the array to compute
fromIndex. If the computed index is less than 0, the whole array will be searched.
When the
includes method is called, the following steps are taken:
Let
n be ?
ToInteger(
fromIndex). (If
fromIndex is
undefined, this step produces the value 0.)
If
n ≥ 0, then
Let
k be
n.
Else
n < 0,
Let
k be
len +
n.
If
k < 0, let
k be 0.
Repeat, while
k <
len
Let
elementK be the result of ?
Get(
O, !
ToString(
k)).
If
SameValueZero(
searchElement,
elementK) is
true, return
true.
Increase
k by 1.
Return
false.
Note 2
The
includes function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Note 3
The
includes method intentionally differs from the similar
indexOf method in two ways. First, it uses the
SameValueZero algorithm, instead of
Strict Equality Comparison, allowing it to detect
NaN array elements. Second, it does not skip missing array elements, instead treating them as
undefined.
indexOf compares
searchElement to the elements of the array, in ascending order, using the
Strict Equality Comparison algorithm, and if found at one or more indices, returns the smallest such index; otherwise, -1 is returned.
The optional second argument
fromIndex defaults to 0 (i.e. the whole array is searched). If it is greater than or equal to the length of
the array, -1 is returned, i.e. the array will not be searched. If it is negative, it is used as the offset from the
end of the array to compute
fromIndex. If the computed index is less than 0, the whole array will be searched.
When the
indexOf method is called with one or two arguments, the following steps are taken:
The
indexOf function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The elements of the array are converted to Strings, and these Strings are then concatenated, separated by occurrences
of the
separator. If no separator is provided, a single comma is used as the separator.
The
join method takes one argument,
separator, and performs the following steps:
If
element is
undefined or
null, let
next be the empty String; otherwise, let
next be ?
ToString(
element).
Let
R be a String value produced by concatenating
S and
next.
Increase
k by 1.
Return
R.
Note 2
The
join function is intentionally generic; it does not require that its
this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
lastIndexOf compares
searchElement to the elements of the array in descending order using the
Strict Equality Comparison algorithm, and if found at one or more indices, returns the largest such index; otherwise, -1 is returned.
The optional second argument
fromIndex defaults to the array's length minus one (i.e. the whole array is searched). If it is greater than
or equal to the length of the array, the whole array will be searched. If it is negative, it is used as the offset
from the end of the array to compute
fromIndex. If the computed index is less than 0, -1 is returned.
When the
lastIndexOf method is called with one or two arguments, the following steps are taken:
The
lastIndexOf function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments.
map calls
callbackfn once for each element in the array, in ascending order, and constructs a new Array from the results.
callbackfn is called only for elements of the array which actually exist; it is not called for missing elements
of the array.
If a
thisArg parameter is provided, it will be used as the
this value for each invocation of
callbackfn. If it is not provided,
undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object
being traversed.
map does not directly mutate the object on which it is called but the object may be mutated by the calls to
callbackfn.
The range of elements processed by
map is set before the first call to
callbackfn. Elements which are appended to the array after the call to
map begins will not be visited by
callbackfn. If existing elements of the array are changed, their value as passed to
callbackfn will be the value at the time
map visits them; elements that are deleted after the call to
map begins and before being visited are not visited.
When the
map method is called with one or two arguments, the following steps are taken:
The
map function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The
pop function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The arguments are appended to the end of the array, in the order in which they appear. The new length of the array
is returned as the result of the call.
When the
push method is called with zero or more arguments, the following steps are taken:
The
push function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that takes four arguments.
reduce calls the callback, as a function, once for each element present in the array, in ascending order.
callbackfn is called with four arguments: the
previousValue (value from the previous call to
callbackfn), the
currentValue (value of the current element), the
currentIndex, and the object being traversed. The first time that callback is called, the
previousValue and
currentValue can be one of two values. If an
initialValue was provided in the call to
reduce, then
previousValue will be equal to
initialValue and
currentValue will be equal to the first value in the array. If no
initialValue was provided, then
previousValue will be equal to the first value in the array and
currentValue will be equal to the second. It is a
TypeError if the array contains no elements and
initialValue is not provided.
reduce does not directly mutate the object on which it is called but the object may be mutated by the calls
to
callbackfn.
The range of elements processed by
reduce is set before the first call to
callbackfn. Elements that are appended to the array after the call to
reduce begins will not be visited by
callbackfn. If existing elements of the array are changed, their value as passed to
callbackfn will be the value at the time
reduce visits them; elements that are deleted after the call to
reduce begins and before being visited are not visited.
When the
reduce method is called with one or two arguments, the following steps are taken:
Let
accumulator be ?
Call(
callbackfn,
undefined, «
accumulator,
kValue,
k,
O »).
Increase
k by 1.
Return
accumulator.
Note 2
The
reduce function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that takes four arguments.
reduceRight calls the callback, as a function, once for each element present in the array, in descending order.
callbackfn is called with four arguments: the
previousValue (value from the previous call to
callbackfn), the
currentValue (value of the current element), the
currentIndex, and the object being traversed. The first time the function is called, the
previousValue and
currentValue can be one of two values. If an
initialValue was provided in the call to
reduceRight, then
previousValue will be equal to
initialValue and
currentValue will be equal to the last value in the array. If no
initialValue was provided, then
previousValue will be equal to the last value in the array and
currentValue will be equal to the second-to-last value. It is a
TypeError if the array contains no elements and
initialValue is not provided.
reduceRight does not directly mutate the object on which it is called but the object may be mutated by the
calls to
callbackfn.
The range of elements processed by
reduceRight is set before the first call to
callbackfn. Elements that are appended to the array after the call to
reduceRight begins will not be visited by
callbackfn. If existing elements of the array are changed by
callbackfn, their value as passed to
callbackfn will be the value at the time
reduceRight visits them; elements that are deleted after the call to
reduceRight begins and before being visited are not visited.
When the
reduceRight method is called with one or two arguments, the following steps are taken:
Let
accumulator be ?
Call(
callbackfn,
undefined, «
accumulator,
kValue,
k,
O »).
Decrease
k by 1.
Return
accumulator.
Note 2
The
reduceRight function is intentionally generic; it does not require that its this value be an Array object.
Therefore it can be transferred to other kinds of objects for use as a method.
The
reverse function is intentionally generic; it does not require that its
this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
The
shift function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The
slice method takes two arguments,
start and
end, and returns an array containing the elements of the array from element
start up to, but not including, element
end (or through the end of the array if
end is
undefined). If
start is negative, it is treated as
length+
start where
length is the length of the array. If
end is negative, it is treated as
length+
end where
length is the length of the array.
The explicit setting of the
length property of the result Array in step 11 was necessary in previous editions of ECMAScript to ensure that
its length was correct in situations where the trailing elements of the result Array were not present. Setting length
became unnecessary starting in ES2015 when the result Array was initialized to its proper length rather than an empty
Array but is carried forward to preserve backward compatibility.
Note 3
The
slice function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean
value
true or
false.
some calls
callbackfn once for each element present in the array, in ascending order, until it finds one where
callbackfn returns
true. If such an element is found,
some immediately returns
true. Otherwise,
some returns
false.
callbackfn is called only for elements of the array which actually exist; it is not called for missing elements
of the array.
If a
thisArg parameter is provided, it will be used as the
this value for each invocation of
callbackfn. If it is not provided,
undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object
being traversed.
some does not directly mutate the object on which it is called but the object may be mutated by the calls to
callbackfn.
The range of elements processed by
some is set before the first call to
callbackfn. Elements that are appended to the array after the call to
some begins will not be visited by
callbackfn. If existing elements of the array are changed, their value as passed to
callbackfn will be the value at the time that
some visits them; elements that are deleted after the call to
some begins and before being visited are not visited.
some acts like the "exists" quantifier in mathematics. In particular, for an empty array, it returns
false.
When the
some method is called with one or two arguments, the following steps are taken:
Let
testResult be
ToBoolean(?
Call(
callbackfn,
T, «
kValue,
k,
O »)).
If
testResult is
true, return
true.
Increase
k by 1.
Return
false.
Note 2
The
some function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The elements of this array are sorted. The sort is not necessarily stable (that is, elements that compare equal do not
necessarily remain in their original order). If
comparefn is not
undefined, it should be a function that accepts two arguments
x and
y and returns a negative value if
x <
y, zero if
x =
y, or a positive value if
x >
y.
Upon entry, the following steps are performed to initialize evaluation of the
sort function:
Within this specification of the
sort method, an object,
obj, is said to be
sparse if the following algorithm returns
true:
For each integer
i in the range 0≤
i<
len
Let
elem be
obj.[[GetOwnProperty]](!
ToString(
i)).
If
elem is
undefined, return
true.
Return
false.
The
sort order is the ordering, after completion of this function, of the integer indexed property values of
obj whose integer indexes are less than
len. The result of the
sort function is then determined as follows:
If
comparefn is not
undefined and is not a consistent comparison function for the elements of this array (see below), the sort
order is implementation-defined. The sort order is also implementation-defined if
comparefn is
undefined and
SortCompare does not act as a consistent comparison function.
Let
proto be
obj.[[GetPrototypeOf]](). If
proto is not
null and there exists an integer
j such that all of the conditions below are satisfied then the sort order is implementation-defined:
Any integer index property of
obj whose name is a nonnegative integer less than
len is a data property whose [[Configurable]] attribute is
false.
The sort order is also implementation defined if any of the following conditions are true:
If
obj is an exotic object (including Proxy exotic objects) whose behaviour for [[Get]], [[Set]], [[Delete]], and
[[GetOwnProperty]] is not the ordinary object implementation of these internal methods.
If any index property of
obj whose name is a nonnegative integer less than
len is an accessor property or is a data property whose [[Writable]] attribute is
false.
If
comparefn is
undefined and the application of
ToString to any value passed as an argument to
SortCompare modifies
obj or any object on
obj's prototype chain.
If
comparefn is
undefined and all applications of
ToString, to any specific value passed as an argument to
SortCompare, do not produce the same result.
The following steps are taken:
Perform an implementation-dependent sequence of calls to the [[Get]] and [[Set]] internal methods of
obj, to the
DeletePropertyOrThrow and
HasOwnProperty abstract operation with
obj as the first argument, and to
SortCompare (described below), such that:
The property key argument for each call to [[Get]], [[Set]],
HasOwnProperty, or
DeletePropertyOrThrow is the string representation of a nonnegative integer less than
len.
The arguments for calls to
SortCompare are values returned by a previous call to the [[Get]] internal method, unless the properties accessed by those
previous calls did not exist according to
HasOwnProperty. If both perspective arguments to
SortCompare correspond to non-existent properties, use
+0 instead of calling
SortCompare. If only the first perspective argument is non-existent use +1. If only the second perspective argument is non-existent
use -1.
If any [[Set]] call returns
false a
TypeError exception is thrown.
If an
abrupt completion is returned from any of these operations, it is immediately returned as the value of this function.
Return
obj.
Unless the sort order is specified above to be implementation-defined, the returned object must have the following two
characteristics:
There must be some mathematical permutation π of the nonnegative integers less than
len, such that for every nonnegative integer
j less than
len, if property
old[
j] existed, then
new[π(
j)] is exactly the same value as
old[
j]. But if property
old[
j] did not exist, then
new[π(
j)] does not exist.
Then for all nonnegative integers
j and
k, each less than
len, if
SortCompare(old[
j], old[
k]) < 0 (see
SortCompare below), then
new[π(
j)] < new[π(
k)].
Here the notation
old[
j] is used to refer to the hypothetical result of calling the [[Get]] internal method of
obj with argument
j before this function is executed, and the notation
new[
j] to refer to the hypothetical result of calling the [[Get]] internal method of
obj with argument
j after this function has been executed.
A function
comparefn is a consistent comparison function for a set of values
S if all of the requirements below are met for all values
a,
b, and
c (possibly the same value) in the set
S: The notation
a <
CFb means
comparefn(
a,
b) < 0;
a =
CFb means
comparefn(
a,
b) = 0 (of either sign); and
a >
CFb means
comparefn(
a,
b) > 0.
Calling
comparefn(
a,
b) always returns the same value
v when given a specific pair of values
a and
b as its two arguments. Furthermore,
Type(
v) is Number, and
v is not
NaN. Note that this implies that exactly one of
a <
CFb,
a =
CFb, and
a >
CFb will be true for a given pair of
a and
b.
Calling
comparefn(
a,
b) does not modify
obj or any object on
obj's prototype chain.
a =
CFa (reflexivity)
If
a =
CFb, then
b =
CFa (symmetry)
If
a =
CFb and
b =
CFc, then
a =
CFc (transitivity of =
CF)
If
a <
CFb and
b <
CFc, then
a <
CFc (transitivity of <
CF)
If
a >
CFb and
b >
CFc, then
a >
CFc (transitivity of >
CF)
Note 1
The above conditions are necessary and sufficient to ensure that
comparefn divides the set
S into equivalence classes and that these equivalence classes are totally ordered.
Note 2
The
sort function is intentionally generic; it does not require that its
this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.25.1Runtime Semantics: SortCompare(
x,
y )
#
The SortCompare abstract operation is called with two arguments
x and
y. It also has access to the
comparefn argument passed to the current invocation of the
sort method. The following steps are taken:
If
x and
y are both
undefined, return
+0.
If
x is
undefined, return 1.
If
y is
undefined, return -1.
If the argument
comparefn is not
undefined, then
Let
v be ?
ToNumber(?
Call(
comparefn,
undefined, «
x,
y »)).
Because non-existent property values always compare greater than
undefined property values, and
undefined always compares greater than any other value,
undefined property values always sort to the end of the result, followed by non-existent property values.
Note 2
Method calls performed by the
ToString abstract operations in steps 5 and 7 have the potential to cause SortCompare to not behave as a consistent comparison
function.
When the
splice method is called with two or more arguments
start,
deleteCount and zero or more
items, the
deleteCount elements of the array starting at integer index
start are replaced by the arguments
items. An Array object containing the deleted elements (if any) is returned.
Perform ?
Set(
A,
"length",
actualDeleteCount,
true).
Let
items be a
List whose elements are, in left to right order, the portion of the actual argument list starting with the third argument.
The list is empty if fewer than three arguments were passed.
Perform ?
Set(
O,
"length",
len -
actualDeleteCount +
itemCount,
true).
Return
A.
Note 2
The explicit setting of the
length property of the result Array in step 19 was necessary in previous editions of ECMAScript to ensure that
its length was correct in situations where the trailing elements of the result Array were not present. Setting
length became unnecessary starting in ES2015 when the result Array was initialized to its proper length rather
than an empty Array but is carried forward to preserve backward compatibility.
Note 3
The
splice function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the
Array.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation
does not include the ECMA-402 API the following specification of the
toLocaleString method is used.
Note 1
The first edition of ECMA-402 did not include a replacement specification for the
Array.prototype.toLocaleString method.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that
do not include ECMA-402 support must not use those parameter positions for anything else.
Let
separator be the String value for the list-separator String appropriate for the host environment's current
locale (this is derived in an implementation-defined way).
Let
R be ?
ToString(?
Invoke(
nextElement,
"toLocaleString")).
Let
R be a String value produced by concatenating
S and
R.
Increase
k by 1.
Return
R.
Note 2
The elements of the array are converted to Strings using their
toLocaleString methods, and these Strings are then concatenated, separated by occurrences of a separator String
that has been derived in an implementation-defined locale-specific way. The result of calling this function is intended
to be analogous to the result of
toString, except that the result of this function is intended to be locale-specific.
Note 3
The
toLocaleString function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The
toString function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The arguments are prepended to the start of the array, such that their order within the array is the same as the order
in which they appear in the argument list.
When the
unshift method is called with zero or more arguments
item1,
item2, etc., the following steps are taken:
The
unshift function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
Note
The own property names of this object are property names that were not included as standard properties of
Array.prototype prior to the ECMAScript 2015 specification. These names are ignored for
with statement binding purposes in order to preserve the behaviour of existing code that might use one of these
names as a binding in an outer scope that is shadowed by a
with statement whose binding object is an Array object.
Array instances are Array exotic objects and have the internal methods specified for such objects. Array instances inherit
properties from the Array prototype object.
Array instances have a
length property, and a set of enumerable properties with array index names.
The
length property of an Array instance is a data property whose value is always numerically greater than the name
of every configurable own property whose name is an array index.
The
length property initially has the attributes { [[Writable]]:
true, [[Enumerable]]:
false, [[Configurable]]:
false }.
Note
Reducing the value of the
length property has the side-effect of deleting own array elements whose array index is between the old and
new length values. However, non-configurable properties can not be deleted. Attempting to set the length property
of an Array object to a value that is numerically less than or equal to the largest numeric own property name of an
existing non-configurable array indexed property of the array will result in the length being set to a numeric value
that is one greater than that non-configurable numeric own property name. See
9.4.2.1.
An Array Iterator is an object, that represents a specific iteration over some specific Array instance object. There
is not a named constructor for Array Iterator objects. Instead, Array iterator objects are created by calling certain
methods of Array instance objects.
Several methods of Array objects return Iterator objects. The abstract operation CreateArrayIterator with arguments
array and
kind is used to create such iterator objects. It performs the following steps:
All Array Iterator Objects inherit properties from the %ArrayIteratorPrototype% intrinsic object. The
%ArrayIteratorPrototype% object is an ordinary object and its [[Prototype]] internal slot is the
%IteratorPrototype% intrinsic object. In addition, %ArrayIteratorPrototype% has the following properties:
Array Iterator instances are ordinary objects that inherit properties from the
%ArrayIteratorPrototype% intrinsic object. Array Iterator instances are initially created with the internal slots listed in
Table 49.
Table 49: Internal Slots of Array Iterator Instances
Internal Slot
Description
[[IteratedObject]]
The object whose array elements are being iterated.
[[ArrayIteratorNextIndex]]
The integer index of the next integer index to be examined by this iteration.
[[ArrayIterationKind]]
A String value that identifies what is returned for each element of the iteration. The possible values are:
"key",
"value",
"key+value".
TypedArray objects present an array-like view of an underlying binary data buffer (
24.1). Each element of a
TypedArray instance has the same underlying binary scalar data type. There is a distinct
TypedArray constructor, listed in
Table 50, for each of the nine supported element types. Each constructor in
Table 50 has a corresponding distinct prototype object.
In the definitions below, references to
TypedArray should be replaced with the appropriate constructor name from the above table. The phrase “the element
size in bytes” refers to the value in the Element Size column of the table in the row corresponding to the constructor.
The phrase “element Type” refers to the value in the Element Type column for that row.
The
%TypedArray% intrinsic object is a constructor function object that all of the
TypedArray constructor object inherit from. %TypedArray% and its corresponding prototype object provide common
properties that are inherited by all
TypedArray constructors and their instances. The %TypedArray% intrinsic does not have a global name or appear
as a property of the
global object.
The %TypedArray% intrinsic function object acts as the abstract superclass of the various
TypedArray constructors. Because it is an abstract class constructor it will throw an error when invoked. The
TypeArray constructors do not perform a super call to it.
%TypedArray%[@@species] is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps:
Return the
this value.
The value of the
name property of this function is
"get [Symbol.species]".
Note
%TypedArrayPrototype% methods normally use their
this object's constructor to create a derived object. However, a subclass constructor may over-ride that default
behaviour by redefining its @@species property.
22.2.3Properties of the %TypedArrayPrototype% Object
#
The value of the [[Prototype]] internal slot of the
%TypedArrayPrototype% object is the intrinsic object
%ObjectPrototype%. The %TypedArrayPrototype% object is an ordinary object. It does not have a [[ViewedArrayBuffer]] or any other of the
internal slots that are specific to
TypedArray instance objects.
%TypedArray%.
prototype.buffer is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps:
Let
O be the
this value.
If
Type(
O) is not Object, throw a
TypeError exception.
If
O does not have a [[ViewedArrayBuffer]] internal slot, throw a
TypeError exception.
Let
buffer be the value of
O's [[ViewedArrayBuffer]] internal slot.
%TypedArray%.
prototype.byteLength is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps:
Let
O be the
this value.
If
Type(
O) is not Object, throw a
TypeError exception.
If
O does not have a [[ViewedArrayBuffer]] internal slot, throw a
TypeError exception.
Let
buffer be the value of
O's [[ViewedArrayBuffer]] internal slot.
%TypedArray%.
prototype.byteOffset is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps:
Let
O be the
this value.
If
Type(
O) is not Object, throw a
TypeError exception.
If
O does not have a [[ViewedArrayBuffer]] internal slot, throw a
TypeError exception.
Let
buffer be the value of
O's [[ViewedArrayBuffer]] internal slot.
22.2.3.5%TypedArray%.prototype.copyWithin (
target,
start [ ,
end ] )
#
%TypedArray%.prototype.copyWithin is a distinct function that implements the same algorithm as
Array.prototype.copyWithin as defined in
22.1.3.3 except that the
this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length" and the actual copying of values in step 12 must be performed in a manner that preserves the bit-level
encoding of the source data
The implementation of the algorithm may be optimized with the knowledge that the
this value is an object that has a fixed length and whose integer indexed properties are not sparse. However,
such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic.
ValidateTypedArray is applied to the
this value prior to evaluating the algorithm. If its result is an
abrupt completion that exception is thrown instead of evaluating the algorithm.
22.2.3.5.1Runtime Semantics: ValidateTypedArray (
O )
#
When called with argument
O, the following steps are taken:
If
Type(
O) is not Object, throw a
TypeError exception.
If
O does not have a [[TypedArrayName]] internal slot, throw a
TypeError exception.
If
O does not have a [[ViewedArrayBuffer]] internal slot, throw a
TypeError exception.
Let
buffer be the value of
O's [[ViewedArrayBuffer]] internal slot.
If
IsDetachedBuffer(
buffer) is
true, throw a
TypeError exception.
%TypedArray%.prototype.every is a distinct function that implements the same algorithm as
Array.prototype.every as defined in
22.1.3.5 except that the
this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm may be optimized with the knowledge that the
this value is an object that has a fixed length and whose integer indexed properties are not sparse. However,
such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take
into account the possibility that calls to
callbackfn may cause the
this value to become detached.
This function is not generic.
ValidateTypedArray is applied to the
this value prior to evaluating the algorithm. If its result is an
abrupt completion that exception is thrown instead of evaluating the algorithm.
22.2.3.8%TypedArray%.prototype.fill (
value [ ,
start [ ,
end ] ] )
#
%TypedArray%.prototype.fill is a distinct function that implements the same algorithm as
Array.prototype.fill as defined in
22.1.3.6 except that the
this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm may be optimized with the knowledge that the
this value is an object that has a fixed length and whose integer indexed properties are not sparse. However,
such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic.
ValidateTypedArray is applied to the
this value prior to evaluating the algorithm. If its result is an
abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.find is a distinct function that implements the same algorithm as
Array.prototype.find as defined in
22.1.3.8 except that the
this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm may be optimized with the knowledge that the
this value is an object that has a fixed length and whose integer indexed properties are not sparse. However,
such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take
into account the possibility that calls to
predicate may cause the
this value to become detached.
This function is not generic.
ValidateTypedArray is applied to the
this value prior to evaluating the algorithm. If its result is an
abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.findIndex is a distinct function that implements the same algorithm as
Array.prototype.findIndex as defined in
22.1.3.9 except that the
this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm may be optimized with the knowledge that the
this value is an object that has a fixed length and whose integer indexed properties are not sparse. However,
such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take
into account the possibility that calls to
predicate may cause the
this value to become detached.
This function is not generic.
ValidateTypedArray is applied to the
this value prior to evaluating the algorithm. If its result is an
abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.forEach is a distinct function that implements the same algorithm as
Array.prototype.forEach as defined in
22.1.3.10 except that the
this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm may be optimized with the knowledge that the
this value is an object that has a fixed length and whose integer indexed properties are not sparse. However,
such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take
into account the possibility that calls to
callbackfn may cause the
this value to become detached.
This function is not generic.
ValidateTypedArray is applied to the
this value prior to evaluating the algorithm. If its result is an
abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.indexOf is a distinct function that implements the same algorithm as
Array.prototype.indexOf as defined in
22.1.3.12 except that the
this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm may be optimized with the knowledge that the
this value is an object that has a fixed length and whose integer indexed properties are not sparse. However,
such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic.
ValidateTypedArray is applied to the
this value prior to evaluating the algorithm. If its result is an
abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.includes is a distinct function that implements the same algorithm as
Array.prototype.includes as defined in
22.1.3.11 except that the
this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm may be optimized with the knowledge that the
this value is an object that has a fixed length and whose integer indexed properties are not sparse. However,
such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic.
ValidateTypedArray is applied to the
this value prior to evaluating the algorithm. If its result is an
abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.join is a distinct function that implements the same algorithm as
Array.prototype.join as defined in
22.1.3.13 except that the
this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm may be optimized with the knowledge that the
this value is an object that has a fixed length and whose integer indexed properties are not sparse. However,
such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic.
ValidateTypedArray is applied to the
this value prior to evaluating the algorithm. If its result is an
abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.lastIndexOf is a distinct function that implements the same algorithm as
Array.prototype.lastIndexOf as defined in
22.1.3.15 except that the
this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm may be optimized with the knowledge that the
this value is an object that has a fixed length and whose integer indexed properties are not sparse. However,
such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic.
ValidateTypedArray is applied to the
this value prior to evaluating the algorithm. If its result is an
abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.
prototype.length is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps:
Let
O be the
this value.
If
Type(
O) is not Object, throw a
TypeError exception.
If
O does not have a [[TypedArrayName]] internal slot, throw a
TypeError exception.
Assert:
O has [[ViewedArrayBuffer]] and [[ArrayLength]] internal slots.
Let
buffer be the value of
O's [[ViewedArrayBuffer]] internal slot.
%TypedArray%.prototype.reduce is a distinct function that implements the same algorithm as
Array.prototype.reduce as defined in
22.1.3.19 except that the
this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm may be optimized with the knowledge that the
this value is an object that has a fixed length and whose integer indexed properties are not sparse. However,
such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take
into account the possibility that calls to
callbackfn may cause the
this value to become detached.
This function is not generic.
ValidateTypedArray is applied to the
this value prior to evaluating the algorithm. If its result is an
abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.reduceRight is a distinct function that implements the same algorithm as
Array.prototype.reduceRight as defined in
22.1.3.20 except that the
this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm may be optimized with the knowledge that the
this value is an object that has a fixed length and whose integer indexed properties are not sparse. However,
such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take
into account the possibility that calls to
callbackfn may cause the
this value to become detached.
This function is not generic.
ValidateTypedArray is applied to the
this value prior to evaluating the algorithm. If its result is an
abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.reverse is a distinct function that implements the same algorithm as
Array.prototype.reverse as defined in
22.1.3.21 except that the
this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm may be optimized with the knowledge that the
this value is an object that has a fixed length and whose integer indexed properties are not sparse. However,
such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic.
ValidateTypedArray is applied to the
this value prior to evaluating the algorithm. If its result is an
abrupt completion that exception is thrown instead of evaluating the algorithm.
Sets multiple values in this
TypedArray, reading the values from the object
array. The optional
offset value indicates the first element index in this
TypedArray where values are written. If omitted, it is assumed to be 0.
Assert:
array is any
ECMAScript language value other than an Object with a [[TypedArrayName]] internal slot. If it is such an Object, the definition in
22.2.3.23.2 applies.
Let
target be the
this value.
If
Type(
target) is not Object, throw a
TypeError exception.
If
target does not have a [[TypedArrayName]] internal slot, throw a
TypeError exception.
Assert:
target has a [[ViewedArrayBuffer]] internal slot.
Sets multiple values in this
TypedArray, reading the values from the
typedArray argument object. The optional
offset value indicates the first element index in this
TypedArray where values are written. If omitted, it is assumed to be 0.
Assert:
typedArray has a [[TypedArrayName]] internal slot. If it does not, the definition in
22.2.3.23.1 applies.
Let
target be the
this value.
If
Type(
target) is not Object, throw a
TypeError exception.
If
target does not have a [[TypedArrayName]] internal slot, throw a
TypeError exception.
Assert:
target has a [[ViewedArrayBuffer]] internal slot.
22.2.3.24%TypedArray%.prototype.slice (
start,
end )
#
The interpretation and use of the arguments of
%TypedArray%.prototype.slice are the same as for
Array.prototype.slice as defined in
22.1.3.23. The following steps are taken:
%TypedArray%.prototype.some is a distinct function that implements the same algorithm as
Array.prototype.some as defined in
22.1.3.24 except that the
this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm may be optimized with the knowledge that the
this value is an object that has a fixed length and whose integer indexed properties are not sparse. However,
such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take
into account the possibility that calls to
callbackfn may cause the
this value to become detached.
This function is not generic.
ValidateTypedArray is applied to the
this value prior to evaluating the algorithm. If its result is an
abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.sort is a distinct function that, except as described below, implements the same requirements as
those of
Array.prototype.sort as defined in
22.1.3.25. The implementation of the
%TypedArray%.prototype.sort specification may be optimized with the knowledge that the
this value is an object that has a fixed length and whose integer indexed properties are not sparse. The
only internal methods of the
this object that the algorithm may call are [[Get]] and [[Set]].
This function is not generic. The
this value must be an object with a [[TypedArrayName]] internal slot.
Upon entry, the following steps are performed to initialize evaluation of the
sort function. These steps are used instead of the entry steps in
22.1.3.25:
Let
len be the value of
obj's [[ArrayLength]] internal slot.
The implementation defined sort order condition for exotic objects is not applied by
%TypedArray%.prototype.sort.
The following version of
SortCompare is used by
%TypedArray%.prototype.sort. It performs a numeric comparison rather than the string comparison used in
22.1.3.25.
SortCompare has access to the
comparefn and
buffer values of the current invocation of the
sort method.
When the TypedArray
SortCompare abstract operation is called with two arguments
x and
y, the following steps are taken:
If
IsDetachedBuffer(
buffer) is
true, throw a
TypeError exception.
If
v is
NaN, return
+0.
Return
v.
If
x and
y are both
NaN, return
+0.
If
x is
NaN, return 1.
If
y is
NaN, return -1.
If
x <
y, return -1.
If
x >
y, return 1.
If
x is
-0 and
y is
+0, return -1.
If
x is
+0 and
y is
-0, return 1.
Return
+0.
Note
Because
NaN always compares greater than any other value,
NaN property values always sort to the end of the result when
comparefn is not provided.
22.2.3.27%TypedArray%.prototype.subarray(
begin,
end )
#
Returns a new
TypedArray object whose element type is the same as this
TypedArray and whose ArrayBuffer is the same as the ArrayBuffer of this
TypedArray, referencing the elements at
begin, inclusive, up to
end, exclusive. If either
begin or
end is negative, it refers to an index from the end of the array, as opposed to from the beginning.
Let
O be the
this value.
If
Type(
O) is not Object, throw a
TypeError exception.
If
O does not have a [[TypedArrayName]] internal slot, throw a
TypeError exception.
Assert:
O has a [[ViewedArrayBuffer]] internal slot.
Let
buffer be the value of
O's [[ViewedArrayBuffer]] internal slot.
Let
srcLength be the value of
O's [[ArrayLength]] internal slot.
%TypedArray%.prototype.toLocaleString is a distinct function that implements the same algorithm as
Array.prototype.toLocaleString as defined in
22.1.3.27 except that the
this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length". The implementation of the algorithm may be optimized with the knowledge that the
this value is an object that has a fixed length and whose integer indexed properties are not sparse. However,
such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic.
ValidateTypedArray is applied to the
this value prior to evaluating the algorithm. If its result is an
abrupt completion that exception is thrown instead of evaluating the algorithm.
Note
If the ECMAScript implementation includes the ECMA-402 Internationalization API this function is based upon the algorithm
for
Array.prototype.toLocaleString that is in the ECMA-402 specification.
The initial value of the
%TypedArray%.prototype.toString data property is the same built-in function object as the
Array.prototype.toString method defined in
22.1.3.28.
%TypedArray%.
prototype[@@toStringTag] is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps:
Each of the
TypedArray constructor objects is an intrinsic object that has the structure described below, differing only in
the name used as the constructor name instead of
TypedArray, in
Table 50.
The
TypedArray intrinsic constructor functions are single functions whose behaviour is overloaded based upon the number
and types of its arguments. The actual behaviour of a call of
TypedArray depends upon the number and kind of arguments that are passed to it.
The
TypedArray constructors are not intended to be called as a function and will throw an exception when called in
that manner.
The
TypedArray constructors are designed to be subclassable. They may be used as the value of an
extends clause of a class definition. Subclass constructors that intend to inherit the specified
TypedArray behaviour must include a
super call to the
TypedArray constructor to create and initialize the subclass instance with the internal state necessary to support
the %TypedArray
%.prototype built-in methods.
The
length property of the
TypedArray constructor function is 3.
The abstract operation AllocateTypedArray with arguments
constructorName,
newTarget,
defaultProto and optional argument
length is used to validate and create an instance of a TypedArray constructor.
constructorName is required to be the name of a TypedArray constructor in
Table 50. If the
length argument is passed an ArrayBuffer of that length is also allocated and associated with the new TypedArray
instance. AllocateTypedArray provides common semantics that is used by all of the
TypedArray overloads. AllocateTypedArray performs the following steps:
22.2.4.2.2Runtime Semantics: AllocateTypedArrayBuffer (
O,
length )
#
The abstract operation AllocateTypedArrayBuffer with arguments
O and
length allocates and associates an ArrayBuffer with the TypedArray instance
O. It performs the following steps:
Assert:
O is an Object that has a [[ViewedArrayBuffer]] internal slot.
Assert: The [[ViewedArrayBuffer]] internal slot of
O is
undefined.
Assert:
length ≥ 0.
Let
constructorName be the String value of
O's [[TypedArrayName]] internal slot.
Let
elementSize be the Element Size value in
Table 50 for
constructorName.
This description applies only if the
TypedArray function is called with at least one argument and the Type of the first argument is Object and that
object has a [[TypedArrayName]] internal slot.
TypedArray called with argument
typedArray performs the following steps:
Assert:
Type(
typedArray) is Object and
typedArray has a [[TypedArrayName]] internal slot.
If NewTarget is
undefined, throw a
TypeError exception.
Let
constructorName be the String value of the Constructor Name value specified in
Table 50 for this
TypedArray constructor.
Let
O be ?
AllocateTypedArray(
constructorName, NewTarget,
"%
TypedArrayPrototype%").
Let
srcArray be
typedArray.
Let
srcData be the value of
srcArray's [[ViewedArrayBuffer]] internal slot.
If
IsDetachedBuffer(
srcData) is
true, throw a
TypeError exception.
Let
constructorName be the String value of
O's [[TypedArrayName]] internal slot.
Let
elementType be the String value of the Element Type value in
Table 50 for
constructorName.
Let
elementLength be the value of
srcArray's [[ArrayLength]] internal slot.
Let
srcName be the String value of
srcArray's [[TypedArrayName]] internal slot.
Let
srcType be the String value of the Element Type value in
Table 50 for
srcName.
Let
srcElementSize be the Element Size value in
Table 50 for
srcName.
Let
srcByteOffset be the value of
srcArray's [[ByteOffset]] internal slot.
Let
elementSize be the Element Size value in
Table 50 for
constructorName.
This description applies only if the
TypedArray function is called with at least one argument and the Type of the first argument is Object and that
object does not have either a [[TypedArrayName]] or an [[ArrayBufferData]] internal slot.
TypedArray called with argument
object performs the following steps:
Assert:
Type(
object) is Object and
object does not have either a [[TypedArrayName]] or an [[ArrayBufferData]] internal slot.
If NewTarget is
undefined, throw a
TypeError exception.
Let
constructorName be the String value of the Constructor Name value specified in
Table 50 for this
TypedArray constructor.
Let
O be ?
AllocateTypedArray(
constructorName, NewTarget,
"%
TypedArrayPrototype%").
This description applies only if the
TypedArray function is called with at least one argument and the Type of the first argument is Object and that
object has an [[ArrayBufferData]] internal slot.
TypedArray called with arguments
buffer,
byteOffset, and
length performs the following steps:
Assert:
Type(
buffer) is Object and
buffer has an [[ArrayBufferData]] internal slot.
If NewTarget is
undefined, throw a
TypeError exception.
Let
constructorName be the String value of the Constructor Name value specified in
Table 50 for this
TypedArray constructor.
Let
O be ?
AllocateTypedArray(
constructorName, NewTarget,
"%
TypedArrayPrototype%").
Let
constructorName be the String value of
O's [[TypedArrayName]] internal slot.
Let
elementSize be the Number value of the Element Size value in
Table 50 for
constructorName.
The abstract operation TypedArrayCreate with arguments
constructor and
argumentList is used to specify the creation of a new TypedArray object using a constructor function. It performs
the following steps:
Let
newTypedArray be ?
Construct(
constructor,
argumentList).
The abstract operation TypedArraySpeciesCreate with arguments
exemplar and
argumentList is used to specify the creation of a new TypedArray object using a constructor function that is
derived from
exemplar. It performs the following steps:
Assert:
exemplar is an Object that has a [[TypedArrayName]] internal slot.
Let
defaultConstructor be the intrinsic object listed in column one of
Table 50 for the value of
exemplar's [[TypedArrayName]] internal slot.
The initial value of
TypedArray.prototype is the corresponding
TypedArray prototype intrinsic object (
22.2.6).
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
22.2.6Properties of
TypedArray Prototype Objects
#
The value of the [[Prototype]] internal slot of a
TypedArray prototype object is the intrinsic object
%TypedArrayPrototype%. A
TypedArray prototype object is an ordinary object. It does not have a [[ViewedArrayBuffer]] or any other of the
internal slots that are specific to
TypedArray instance objects.
TypedArray instances are Integer Indexed exotic objects. Each
TypedArray instance inherits properties from the corresponding
TypedArray prototype object. Each
TypedArray instance has the following internal slots: [[TypedArrayName]], [[ViewedArrayBuffer]], [[ByteLength]],
[[ByteOffset]], and [[ArrayLength]].
Map objects are collections of key/value pairs where both the keys and values may be arbitrary ECMAScript language values.
A distinct key value may only occur in one key/value pair within the Map's collection. Distinct key values are discriminated
using the
SameValueZero comparison algorithm.
Map object must be implemented using either hash tables or other mechanisms that, on average, provide access times that
are sublinear on the number of elements in the collection. The data structures used in this Map objects specification
is only intended to describe the required observable semantics of Map objects. It is not intended to be a viable implementation
model.
The Map constructor is the
%Map% intrinsic object and the initial value of the
Map property of the
global object. When called as a constructor it creates and initializes a new Map object.
Map is not intended to be called as a function and will throw an exception when called in that manner.
The
Map constructor is designed to be subclassable. It may be used as the value in an
extends clause of a class definition. Subclass constructors that intend to inherit the specified
Map behaviour must include a
super call to the
Map constructor to create and initialize the subclass instance with the internal state necessary to support the
Map.prototype built-in methods.
If the parameter
iterable is present, it is expected to be an object that implements an @@iterator method that returns an iterator
object that produces a two element array-like object whose first element is a value that will be used as a Map key
and whose second element is the value to associate with that key.
Map[@@species] is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps:
Return the
this value.
The value of the
name property of this function is
"get [Symbol.species]".
Note
Methods that create derived collection objects should call @@species to determine the constructor to use to create
the derived objects. Subclass constructor may over-ride @@species to change the default constructor assignment.
The Map prototype object is the intrinsic object
%MapPrototype%. The value of the [[Prototype]] internal slot of the Map prototype object is the intrinsic object
%ObjectPrototype%. The Map prototype object is an ordinary object. It does not have a [[MapData]] internal slot.
If
Type(
M) is not Object, throw a
TypeError exception.
If
M does not have a [[MapData]] internal slot, throw a
TypeError exception.
Let
entries be the
List that is the value of
M's [[MapData]] internal slot.
Repeat for each
Record {[[Key]], [[Value]]}
p that is an element of
entries,
Set
p.[[Key]] to
empty.
Set
p.[[Value]] to
empty.
Return
undefined.
Note
The existing [[MapData]]
List is preserved because there may be existing Map Iterator objects that are suspended midway through iterating over that
List.
If
Type(
M) is not Object, throw a
TypeError exception.
If
M does not have a [[MapData]] internal slot, throw a
TypeError exception.
Let
entries be the
List that is the value of
M's [[MapData]] internal slot.
Repeat for each
Record {[[Key]], [[Value]]}
p that is an element of
entries,
If
p.[[Key]] is not
empty and
SameValueZero(
p.[[Key]],
key) is
true, then
Set
p.[[Key]] to
empty.
Set
p.[[Value]] to
empty.
Return
true.
Return
false.
Note
The value
empty is used as a specification device to indicate that an entry has been deleted. Actual implementations
may take other actions such as physically removing the entry from internal data structures.
When the
forEach method is called with one or two arguments, the following steps are taken:
Let
M be the
this value.
If
Type(
M) is not Object, throw a
TypeError exception.
If
M does not have a [[MapData]] internal slot, throw a
TypeError exception.
If
IsCallable(
callbackfn) is
false, throw a
TypeError exception.
If
thisArg was supplied, let
T be
thisArg; else let
T be
undefined.
Let
entries be the
List that is the value of
M's [[MapData]] internal slot.
Repeat for each
Record {[[Key]], [[Value]]}
e that is an element of
entries, in original key insertion order
If
e.[[Key]] is not
empty, then
Perform ?
Call(
callbackfn,
T, «
e.[[Value]],
e.[[Key]],
M »).
Return
undefined.
Note
callbackfn should be a function that accepts three arguments.
forEach calls
callbackfn once for each key/value pair present in the map object, in key insertion order.
callbackfn is called only for keys of the map which actually exist; it is not called for keys that have been
deleted from the map.
If a
thisArg parameter is provided, it will be used as the
this value for each invocation of
callbackfn. If it is not provided,
undefined is used instead.
callbackfn is called with three arguments: the value of the item, the key of the item, and the Map object being
traversed.
forEach does not directly mutate the object on which it is called but the object may be mutated by the calls
to
callbackfn. Each entry of a map's [[MapData]] is only visited once. New keys added after the call to
forEach begins are visited. A key will be revisited if it is deleted after it has been visited and then re-added
before the
forEach call completes. Keys that are deleted after the call to
forEach begins and before being visited are not visited unless the key is added again before the
forEach call completes.
A Map Iterator is an object, that represents a specific iteration over some specific Map instance object. There is not
a named constructor for Map Iterator objects. Instead, map iterator objects are created by calling certain methods of
Map instance objects.
Several methods of Map objects return Iterator objects. The abstract operation CreateMapIterator with arguments
map and
kind is used to create such iterator objects. It performs the following steps:
If
Type(
map) is not Object, throw a
TypeError exception.
If
map does not have a [[MapData]] internal slot, throw a
TypeError exception.
All Map Iterator Objects inherit properties from the
%MapIteratorPrototype% intrinsic object. The %MapIteratorPrototype% intrinsic object is an ordinary object and
its [[Prototype]] internal slot is the
%IteratorPrototype% intrinsic object. In addition, %MapIteratorPrototype% has the following properties:
Let
entries be the
List that is the value of the [[MapData]] internal slot of
m.
Repeat while
index is less than the total number of elements of
entries. The number of elements must be redetermined each time this method is evaluated.
Let
e be the
Record {[[Key]], [[Value]]} that is the value of
entries[
index].
Set
index to
index+1.
Set the [[MapNextIndex]] internal slot of
O to
index.
If
e.[[Key]] is not
empty, then
If
itemKind is
"key", let
result be
e.[[Key]].
Else if
itemKind is
"value", let
result be
e.[[Value]].
Map Iterator instances are ordinary objects that inherit properties from the
%MapIteratorPrototype% intrinsic object. Map Iterator instances are initially created with the internal slots described in
Table 51.
Table 51: Internal Slots of Map Iterator Instances
Internal Slot
Description
[[Map]]
The Map object that is being iterated.
[[MapNextIndex]]
The integer index of the next Map data element to be examined by this iterator.
[[MapIterationKind]]
A String value that identifies what is to be returned for each element of the iteration. The possible values are:
"key",
"value",
"key+value".
Set objects are collections of ECMAScript language values. A distinct value may only occur once as an element of a Set's
collection. Distinct values are discriminated using the
SameValueZero comparison algorithm.
Set objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that
are sublinear on the number of elements in the collection. The data structures used in this Set objects specification
is only intended to describe the required observable semantics of Set objects. It is not intended to be a viable implementation
model.
The Set constructor is the
%Set% intrinsic object and the initial value of the
Set property of the
global object. When called as a constructor it creates and initializes a new Set object.
Set is not intended to be called as a function and will throw an exception when called in that manner.
The
Set constructor is designed to be subclassable. It may be used as the value in an
extends clause of a class definition. Subclass constructors that intend to inherit the specified
Set behaviour must include a
super call to the
Set constructor to create and initialize the subclass instance with the internal state necessary to support the
Set.prototype built-in methods.
Set[@@species] is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps:
Return the
this value.
The value of the
name property of this function is
"get [Symbol.species]".
Note
Methods that create derived collection objects should call @@species to determine the constructor to use to create
the derived objects. Subclass constructor may over-ride @@species to change the default constructor assignment.
The Set prototype object is the intrinsic object
%SetPrototype%. The value of the [[Prototype]] internal slot of the Set prototype object is the intrinsic object
%ObjectPrototype%. The Set prototype object is an ordinary object. It does not have a [[SetData]] internal slot.
If
Type(
S) is not Object, throw a
TypeError exception.
If
S does not have a [[SetData]] internal slot, throw a
TypeError exception.
Let
entries be the
List that is the value of
S's [[SetData]] internal slot.
Repeat for each
e that is an element of
entries,
Replace the element of
entries whose value is
e with an element whose value is
empty.
Return
undefined.
Note
The existing [[SetData]]
List is preserved because there may be existing Set Iterator objects that are suspended midway through iterating over that
List.
If
Type(
S) is not Object, throw a
TypeError exception.
If
S does not have a [[SetData]] internal slot, throw a
TypeError exception.
Let
entries be the
List that is the value of
S's [[SetData]] internal slot.
Repeat for each
e that is an element of
entries,
If
e is not
empty and
SameValueZero(
e,
value) is
true, then
Replace the element of
entries whose value is
e with an element whose value is
empty.
Return
true.
Return
false.
Note
The value
empty is used as a specification device to indicate that an entry has been deleted. Actual implementations
may take other actions such as physically removing the entry from internal data structures.
callbackfn should be a function that accepts three arguments.
forEach calls
callbackfn once for each value present in the set object, in value insertion order.
callbackfn is called only for values of the Set which actually exist; it is not called for keys that have been
deleted from the set.
If a
thisArg parameter is provided, it will be used as the
this value for each invocation of
callbackfn. If it is not provided,
undefined is used instead.
callbackfn is called with three arguments: the first two arguments are a value contained in the Set. The same
value is passed for both arguments. The Set object being traversed is passed as the third argument.
The
callbackfn is called with three arguments to be consistent with the call back functions used by
forEach methods for Map and Array. For Sets, each item value is considered to be both the key and the value.
forEach does not directly mutate the object on which it is called but the object may be mutated by the calls
to
callbackfn.
Each value is normally visited only once. However, a value will be revisited if it is deleted after it has been visited
and then re-added before the
forEach call completes. Values that are deleted after the call to
forEach begins and before being visited are not visited unless the value is added again before the
forEach call completes. New values added after the call to
forEach begins are visited.
A Set Iterator is an ordinary object, with the structure defined below, that represents a specific iteration over some
specific Set instance object. There is not a named constructor for Set Iterator objects. Instead, set iterator objects
are created by calling certain methods of Set instance objects.
Several methods of Set objects return Iterator objects. The abstract operation CreateSetIterator with arguments
set and
kind is used to create such iterator objects. It performs the following steps:
If
Type(
set) is not Object, throw a
TypeError exception.
If
set does not have a [[SetData]] internal slot, throw a
TypeError exception.
All Set Iterator Objects inherit properties from the
%SetIteratorPrototype% intrinsic object. The %SetIteratorPrototype% intrinsic object is an ordinary object and
its [[Prototype]] internal slot is the
%IteratorPrototype% intrinsic object. In addition, %SetIteratorPrototype% has the following properties:
Let
entries be the
List that is the value of the [[SetData]] internal slot of
s.
Repeat while
index is less than the total number of elements of
entries. The number of elements must be redetermined each time this method is evaluated.
Let
e be
entries[
index].
Set
index to
index+1.
Set the [[SetNextIndex]] internal slot of
O to
index.
Set Iterator instances are ordinary objects that inherit properties from the
%SetIteratorPrototype% intrinsic object. Set Iterator instances are initially created with the internal slots specified in
Table 52.
Table 52: Internal Slots of Set Iterator Instances
Internal Slot
Description
[[IteratedSet]]
The Set object that is being iterated.
[[SetNextIndex]]
The integer index of the next Set data element to be examined by this iterator
[[SetIterationKind]]
A String value that identifies what is to be returned for each element of the iteration. The possible values are:
"key",
"value",
"key+value".
"key" and
"value" have the same meaning.
WeakMap objects are collections of key/value pairs where the keys are objects and values may be arbitrary ECMAScript language
values. A WeakMap may be queried to see if it contains a key/value pair with a specific key, but no mechanism is provided
for enumerating the objects it holds as keys. If an object that is being used as the key of a WeakMap key/value pair
is only reachable by following a chain of references that start within that WeakMap, then that key/value pair is inaccessible
and is automatically removed from the WeakMap. WeakMap implementations must detect and remove such key/value pairs and
any associated resources.
An implementation may impose an arbitrarily determined latency between the time a key/value pair of a WeakMap becomes
inaccessible and the time when the key/value pair is removed from the WeakMap. If this latency was observable to ECMAScript
program, it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation
must not provide any means to observe a key of a WeakMap that does not require the observer to present the observed key.
WeakMap objects must be implemented using either hash tables or other mechanisms that, on average, provide access times
that are sublinear on the number of key/value pairs in the collection. The data structure used in this WeakMap objects
specification are only intended to describe the required observable semantics of WeakMap objects. It is not intended
to be a viable implementation model.
Note
WeakMap and WeakSets are intended to provide mechanisms for dynamically associating state with an object in a manner
that does not “leak” memory resources if, in the absence of the WeakMap or WeakSet, the object otherwise became inaccessible
and subject to resource reclamation by the implementation's garbage collection mechanisms. This characteristic can be
achieved by using an inverted per-object mapping of weak map instances to keys. Alternatively each weak map may internally
store its key to value mappings but this approach requires coordination between the WeakMap or WeakSet implementation
and the garbage collector. The following references describe mechanism that may be useful to implementations of WeakMap
and WeakSets:
Barry Hayes. 1997. Ephemerons: a new finalization mechanism. In
Proceedings of the 12th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications
(OOPSLA '97), A. Michael Berman (Ed.). ACM, New York, NY, USA, 176-183,
http://doi.acm.org/10.1145/263698.263733.
The WeakMap constructor is the
%WeakMap% intrinsic object and the initial value of the
WeakMap property of the
global object. When called as a constructor it creates and initializes a new WeakMap object.
WeakMap is not intended to be called as a function and will throw an exception when called in that manner.
The
WeakMap constructor is designed to be subclassable. It may be used as the value in an
extends clause of a class definition. Subclass constructors that intend to inherit the specified
WeakMap behaviour must include a
super call to the
WeakMap constructor to create and initialize the subclass instance with the internal state necessary to support
the
WeakMap.prototype built-in methods.
If the parameter
iterable is present, it is expected to be an object that implements an @@iterator method that returns an iterator
object that produces a two element array-like object whose first element is a value that will be used as a WeakMap
key and whose second element is the value to associate with that key.
The initial value of
WeakMap.prototype is the intrinsic object
%WeakMapPrototype%.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
23.3.3Properties of the WeakMap Prototype Object
#
The WeakMap prototype object is the intrinsic object
%WeakMapPrototype%. The value of the [[Prototype]] internal slot of the WeakMap prototype object is the intrinsic
object
%ObjectPrototype%. The WeakMap prototype object is an ordinary object. It does not have a [[WeakMapData]] internal slot.
Repeat for each
Record {[[Key]], [[Value]]}
p that is an element of
entries,
If
p.[[Key]] is not
empty and
SameValue(
p.[[Key]],
key) is
true, then
Set
p.[[Key]] to
empty.
Set
p.[[Value]] to
empty.
Return
true.
Return
false.
Note
The value
empty is used as a specification device to indicate that an entry has been deleted. Actual implementations
may take other actions such as physically removing the entry from internal data structures.
WeakMap instances are ordinary objects that inherit properties from the WeakMap prototype. WeakMap instances also have
a [[WeakMapData]] internal slot.
WeakSet objects are collections of objects. A distinct object may only occur once as an element of a WeakSet's collection.
A WeakSet may be queried to see if it contains a specific object, but no mechanism is provided for enumerating the objects
it holds. If an object that is contained by a WeakSet is only reachable by following a chain of references that start
within that WeakSet, then that object is inaccessible and is automatically removed from the WeakSet. WeakSet implementations
must detect and remove such objects and any associated resources.
An implementation may impose an arbitrarily determined latency between the time an object contained in a WeakSet becomes
inaccessible and the time when the object is removed from the WeakSet. If this latency was observable to ECMAScript program,
it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation
must not provide any means to determine if a WeakSet contains a particular object that does not require the observer
to present the observed object.
WeakSet objects must be implemented using either hash tables or other mechanisms that, on average, provide access times
that are sublinear on the number of elements in the collection. The data structure used in this WeakSet objects specification
is only intended to describe the required observable semantics of WeakSet objects. It is not intended to be a viable
implementation model.
The WeakSet constructor is the
%WeakSet% intrinsic object and the initial value of the
WeakSet property of the
global object. When called as a constructor it creates and initializes a new WeakSet object.
WeakSet is not intended to be called as a function and will throw an exception when called in that manner.
The
WeakSet constructor is designed to be subclassable. It may be used as the value in an
extends clause of a class definition. Subclass constructors that intend to inherit the specified
WeakSet behaviour must include a
super call to the
WeakSet constructor to create and initialize the subclass instance with the internal state necessary to support
the
WeakSet.prototype built-in methods.
The initial value of
WeakSet.prototype is the intrinsic
%WeakSetPrototype% object.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
23.4.3Properties of the WeakSet Prototype Object
#
The WeakSet prototype object is the intrinsic object
%WeakSetPrototype%. The value of the [[Prototype]] internal slot of the WeakSet prototype object is the intrinsic
object
%ObjectPrototype%. The WeakSet prototype object is an ordinary object. It does not have a [[WeakSetData]] internal slot.
Let
entries be the
List that is the value of
S's [[WeakSetData]] internal slot.
Repeat for each
e that is an element of
entries,
If
e is not
empty and
SameValue(
e,
value) is
true, then
Replace the element of
entries whose value is
e with an element whose value is
empty.
Return
true.
Return
false.
Note
The value
empty is used as a specification device to indicate that an entry has been deleted. Actual implementations
may take other actions such as physically removing the entry from internal data structures.
WeakSet instances are ordinary objects that inherit properties from the WeakSet prototype. WeakSet instances also have
a [[WeakSetData]] internal slot.
The abstract operation AllocateArrayBuffer with arguments
constructor and
byteLength is used to create an ArrayBuffer object. It performs the following steps:
Let
obj be ?
OrdinaryCreateFromConstructor(
constructor,
"%ArrayBufferPrototype%", « [[ArrayBufferData]], [[ArrayBufferByteLength]] »).
Detaching an ArrayBuffer instance disassociates the
Data Block used as its backing store from the instance and sets the byte length of the buffer to 0. No operations defined by
this specification use the DetachArrayBuffer abstract operation. However, an ECMAScript implementation or host environment
may define such operations.
The abstract operation CloneArrayBuffer takes three parameters, an ArrayBuffer
srcBuffer, an integer
srcByteOffset and optionally a constructor function
cloneConstructor. It creates a new ArrayBuffer whose data is a copy of
srcBuffer's data starting at
srcByteOffset. This operation performs the following steps:
Assert:
Type(
srcBuffer) is Object and it has an [[ArrayBufferData]] internal slot.
The abstract operation GetValueFromBuffer takes four parameters, an ArrayBuffer
arrayBuffer, an integer
byteIndex, a String
type, and optionally a Boolean
isLittleEndian. This operation performs the following steps:
Assert: There are sufficient bytes in
arrayBuffer starting at
byteIndex to represent a value of
type.
Assert:
byteIndex is an integer value ≥ 0.
Let
block be
arrayBuffer's [[ArrayBufferData]] internal slot.
Let
elementSize be the Number value of the Element Size value specified in
Table 50 for Element Type
type.
Let
rawValue be a
List of
elementSize containing, in order, the
elementSize sequence of bytes starting with
block[
byteIndex].
If
isLittleEndian is not present, set
isLittleEndian to either
true or
false. The choice is implementation dependent and should be the alternative that is most efficient for
the implementation. An implementation must use the same value each time this step is executed and the same value
must be used for the corresponding step in the
SetValueInBuffer abstract operation.
If
isLittleEndian is
false, reverse the order of the elements of
rawValue.
If
type is
"Float32", then
Let
value be the byte elements of
rawValue concatenated and interpreted as a little-endian bit string encoding of an IEEE 754-2008 binary32
value.
If
value is an IEEE 754-2008 binary32 NaN value, return the
NaN Number value.
Return the Number value that corresponds to
value.
If
type is
"Float64", then
Let
value be the byte elements of
rawValue concatenated and interpreted as a little-endian bit string encoding of an IEEE 754-2008 binary64
value.
If
value is an IEEE 754-2008 binary64 NaN value, return the
NaN Number value.
Return the Number value that corresponds to
value.
If the first code unit of
type is
"U", then
Let
intValue be the byte elements of
rawValue concatenated and interpreted as a bit string encoding of an unsigned little-endian binary number.
Else,
Let
intValue be the byte elements of
rawValue concatenated and interpreted as a bit string encoding of a binary little-endian 2's complement number
of bit length
elementSize × 8.
Return the Number value that corresponds to
intValue.
The abstract operation SetValueInBuffer takes five parameters, an ArrayBuffer
arrayBuffer, an integer
byteIndex, a String
type, a Number
value, and optionally a Boolean
isLittleEndian. This operation performs the following steps:
Let
block be
arrayBuffer's [[ArrayBufferData]] internal slot.
Assert:
block is not
undefined.
If
isLittleEndian is not present, set
isLittleEndian to either
true or
false. The choice is implementation dependent and should be the alternative that is most efficient for
the implementation. An implementation must use the same value each time this step is executed and the same value
must be used for the corresponding step in the
GetValueFromBuffer abstract operation.
If
type is
"Float32", then
Set
rawBytes to a
List containing the 4 bytes that are the result of converting
value to IEEE 754-2008 binary32 format using “Round to nearest, ties to even” rounding mode. If
isLittleEndian is
false, the bytes are arranged in big endian order. Otherwise, the bytes are arranged in little endian
order. If
value is
NaN,
rawValue may be set to any implementation chosen IEEE 754-2008 binary64 format Not-a-Number encoding. An
implementation must always choose the same encoding for each implementation distinguishable
NaN value.
Else, if
type is
"Float64", then
Set
rawBytes to a
List containing the 8 bytes that are the IEEE 754-2008 binary64 format encoding of
value. If
isLittleEndian is
false, the bytes are arranged in big endian order. Otherwise, the bytes are arranged in little endian
order. If
value is
NaN,
rawValue may be set to any implementation chosen IEEE 754-2008 binary32 format Not-a-Number encoding. An
implementation must always choose the same encoding for each implementation distinguishable
NaN value.
Else,
Let
n be the Number value of the Element Size specified in
Table 50 for Element Type
type.
Let
convOp be the abstract operation named in the Conversion Operation column in
Table 50 for Element Type
type.
Let
intValue be
convOp(
value).
If
intValue ≥ 0, then
Let
rawBytes be a
List containing the
n-byte binary encoding of
intValue. If
isLittleEndian is
false, the bytes are ordered in big endian order. Otherwise, the bytes are ordered in little endian
order.
Else,
Let
rawBytes be a
List containing the
n-byte binary 2's complement encoding of
intValue. If
isLittleEndian is
false, the bytes are ordered in big endian order. Otherwise, the bytes are ordered in little endian
order.
Store the individual bytes of
rawBytes into
block, in order, starting at
block[
byteIndex].
The ArrayBuffer constructor is the
%ArrayBuffer% intrinsic object and the initial value of the
ArrayBuffer property of the
global object. When called as a constructor it creates and initializes a new ArrayBuffer object.
ArrayBuffer is not intended to be called as a function and will throw an exception when called in that manner.
The
ArrayBuffer constructor is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass constructors that intend to inherit the specified
ArrayBuffer behaviour must include a
super call to the
ArrayBuffer constructor to create and initialize subclass instances with the internal state necessary to support
the
ArrayBuffer.prototype built-in methods.
ArrayBuffer[@@species] is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps:
Return the
this value.
The value of the
name property of this function is
"get [Symbol.species]".
Note
ArrayBuffer prototype methods normally use their
this object's constructor to create a derived object. However, a subclass constructor may over-ride that default
behaviour by redefining its @@species property.
24.1.4Properties of the ArrayBuffer Prototype Object
#
The ArrayBuffer prototype object is the intrinsic object
%ArrayBufferPrototype%. The value of the [[Prototype]] internal slot of the ArrayBuffer prototype object is the
intrinsic object
%ObjectPrototype%. The ArrayBuffer prototype object is an ordinary object. It does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]]
internal slot.
ArrayBuffer.prototype.byteLength is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps:
Let
O be the
this value.
If
Type(
O) is not Object, throw a
TypeError exception.
If
O does not have an [[ArrayBufferData]] internal slot, throw a
TypeError exception.
ArrayBuffer instances inherit properties from the ArrayBuffer prototype object. ArrayBuffer instances each have an [[ArrayBufferData]]
internal slot and an [[ArrayBufferByteLength]] internal slot.
ArrayBuffer instances whose [[ArrayBufferData]] is
null are considered to be detached and all operators to access or modify data contained in the ArrayBuffer
instance will fail.
24.2.1.1GetViewValue (
view,
requestIndex,
isLittleEndian,
type )
#
The abstract operation GetViewValue with arguments
view,
requestIndex,
isLittleEndian, and
type is used by functions on DataView instances is to retrieve values from the view's buffer. It performs the
following steps:
If
Type(
view) is not Object, throw a
TypeError exception.
If
view does not have a [[DataView]] internal slot, throw a
TypeError exception.
24.2.1.2SetViewValue (
view,
requestIndex,
isLittleEndian,
type,
value )
#
The abstract operation SetViewValue with arguments
view,
requestIndex,
isLittleEndian,
type, and
value is used by functions on DataView instances to store values into the view's buffer. It performs the following
steps:
If
Type(
view) is not Object, throw a
TypeError exception.
If
view does not have a [[DataView]] internal slot, throw a
TypeError exception.
The DataView constructor is the
%DataView% intrinsic object and the initial value of the
DataView property of the
global object. When called as a constructor it creates and initializes a new DataView object.
DataView is not intended to be called as a function and will throw an exception when called in that manner.
The
DataView constructor is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass constructors that intend to inherit the specified
DataView behaviour must include a
super call to the
DataView constructor to create and initialize subclass instances with the internal state necessary to support
the
DataView.prototype built-in methods.
The initial value of
DataView.prototype is the intrinsic object
%DataViewPrototype%.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
24.2.4Properties of the DataView Prototype Object
#
The DataView prototype object is the intrinsic object
%DataViewPrototype%. The value of the [[Prototype]] internal slot of the DataView prototype object is the intrinsic
object
%ObjectPrototype%. The DataView prototype object is an ordinary object. It does not have a [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]],
or [[ByteOffset]] internal slot.
DataView.
prototype.byteLength is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps:
Let
O be the
this value.
If
Type(
O) is not Object, throw a
TypeError exception.
If
O does not have a [[ViewedArrayBuffer]] internal slot, throw a
TypeError exception.
Let
buffer be the value of
O's [[ViewedArrayBuffer]] internal slot.
If
IsDetachedBuffer(
buffer) is
true, throw a
TypeError exception.
Let
size be the value of
O's [[ByteLength]] internal slot.
DataView.
prototype.byteOffset is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps:
Let
O be the
this value.
If
Type(
O) is not Object, throw a
TypeError exception.
If
O does not have a [[ViewedArrayBuffer]] internal slot, throw a
TypeError exception.
Let
buffer be the value of
O's [[ViewedArrayBuffer]] internal slot.
If
IsDetachedBuffer(
buffer) is
true, throw a
TypeError exception.
Let
offset be the value of
O's [[ByteOffset]] internal slot.
DataView instances are ordinary objects that inherit properties from the DataView prototype object. DataView instances
each have [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], and [[ByteOffset]] internal slots.
Note
The value of the [[DataView]] internal slot is not used within this specification. The simple presence of that internal
slot is used within the specification to identify objects created using the
DataView constructor.
The JSON object is the
%JSON% intrinsic object and the initial value of the
JSON property of the
global object. The JSON object is a single ordinary object that contains two functions,
parse and
stringify, that are used to parse and construct JSON texts. The JSON Data Interchange Format is defined in ECMA-404.
The JSON interchange format used in this specification is exactly that described by ECMA-404.
Conforming implementations of
JSON.parse and
JSON.stringify must support the exact interchange format described in the ECMA-404 specification without any deletions
or extensions to the format.
The value of the [[Prototype]] internal slot of the JSON object is the intrinsic object
%ObjectPrototype%. The value of the [[Extensible]] internal slot of the JSON object is set to
true.
The JSON object does not have a [[Construct]] internal method; it is not possible to use the JSON object as a constructor
with the
new operator.
The JSON object does not have a [[Call]] internal method; it is not possible to invoke the JSON object as a function.
The
parse function parses a JSON text (a JSON-formatted String) and produces an ECMAScript value. The JSON format
is a subset of the syntax for ECMAScript literals, Array Initializers and Object Initializers. After parsing, JSON objects
are realized as ECMAScript objects. JSON arrays are realized as ECMAScript Array instances. JSON strings, numbers, booleans,
and null are realized as ECMAScript Strings, Numbers, Booleans, and
null.
The optional
reviver parameter is a function that takes two parameters,
key and
value. It can filter and transform the results. It is called with each of the
key/
value pairs produced by the parse, and its return value is used instead of the original value. If it returns what
it received, the structure is not modified. If it returns
undefined then the property is deleted from the result.
Parse
JText interpreted as UTF-16 encoded Unicode points (
6.1.4) as a JSON text as specified in ECMA-404. Throw a
SyntaxError exception if
JText is not a valid JSON text as defined in that specification.
Let
scriptText be the result of concatenating
"(",
JText, and
");".
Let
completion be the result of parsing and evaluating
scriptText as if it was the source text of an ECMAScript
Script, but using the alternative definition of
DoubleStringCharacter provided below. The extended PropertyDefinitionEvaluation semantics defined in
B.3.1 must not be used during the evaluation.
Let
unfiltered be
completion.[[Value]].
Assert:
unfiltered will be either a primitive value or an object that is defined by either an
ArrayLiteral or an
ObjectLiteral.
JSON allows Unicode code units 0x2028 (LINE SEPARATOR) and 0x2029 (PARAGRAPH SEPARATOR) to directly appear in String
literals without using an escape sequence. This is enabled by using the following alternative definition of
DoubleStringCharacter when parsing
scriptText in step 4:
The syntax of a valid JSON text is a subset of the ECMAScript
PrimaryExpression syntax. Hence a valid JSON text is also a valid
PrimaryExpression. Step 2 above verifies that
JText conforms to that subset. When
scriptText is parsed and evaluated as a
Script the result will be either a String, Number, Boolean, or Null primitive value or an Object defined as if by an
ArrayLiteral or
ObjectLiteral.
The abstract operation InternalizeJSONProperty is a recursive abstract operation that takes two parameters: a
holder object and the String
name of a property in that object. InternalizeJSONProperty uses the value of
reviver that was originally passed to the above parse function.
It is not permitted for a conforming implementation of
JSON.parse to extend the JSON grammars. If an implementation wishes to support a modified or extended JSON interchange
format it must do so by defining a different parse function.
Note
In the case where there are duplicate name Strings within an object, lexically preceding values for the same key shall
be overwritten.
24.3.2JSON.stringify (
value [ ,
replacer [ ,
space ] ] )
#
The
stringify function returns a String in UTF-16 encoded JSON format representing an ECMAScript value. It can take
three parameters. The
value parameter is an ECMAScript value, which is usually an object or array, although it can also be a String,
Boolean, Number or
null. The optional
replacer parameter is either a function that alters the way objects and arrays are stringified, or an array of
Strings and Numbers that acts as a white list for selecting the object properties that will be stringified. The optional
space parameter is a String or Number that allows the result to have white space injected into it to improve human
readability.
The
length property of the
stringify function is 3.
Note 1
JSON structures are allowed to be nested to any depth, but they must be acyclic. If
value is or contains a cyclic structure, then the stringify function must throw a
TypeError exception. This is an example of a value that cannot be stringified:
a = [];
a[0] = a;
my_text = JSON.stringify(a); // This must throw a TypeError.
Note 2
Symbolic primitive values are rendered as follows:
The
null value is rendered in JSON text as the String
null.
The
undefined value is not rendered.
The
true value is rendered in JSON text as the String
true.
The
false value is rendered in JSON text as the String
false.
Note 3
String values are wrapped in QUOTATION MARK (
") code units. The code units
" and
\ are escaped with
\ prefixes. Control characters code units are replaced with escape sequences
\uHHHH, or with the shorter forms,
\b (BACKSPACE),
\f (FORM FEED),
\n (LINE FEED),
\r (CARRIAGE RETURN),
\t (CHARACTER TABULATION).
Note 4
Finite numbers are stringified as if by calling
ToString(
number).
NaN and Infinity regardless of sign are represented as the String
null.
Note 5
Values that do not have a JSON representation (such as
undefined and functions) do not produce a String. Instead they produce the
undefined value. In arrays these values are represented as the String
null. In objects an unrepresentable value causes the property to be excluded from stringification.
Note 6
An object is rendered as U+007B (LEFT CURLY BRACKET) followed by zero or more properties, separated with a U+002C (COMMA),
closed with a U+007D (RIGHT CURLY BRACKET). A property is a quoted String representing the key or property name, a
U+003A (COLON), and then the stringified property value. An array is rendered as an opening U+005B (LEFT SQUARE BRACKET
followed by zero or more values, separated with a U+002C (COMMA), closed with a U+005D (RIGHT SQUARE BRACKET).
The abstract operation SerializeJSONProperty with arguments
key, and
holder has access to
ReplacerFunction from the invocation of the
stringify method. Its algorithm is as follows:
24.3.2.2Runtime Semantics: QuoteJSONString (
value )
#
The abstract operation QuoteJSONString with argument
value wraps a String value in QUOTATION MARK code units and escapes certain other code units within it.
Let
product be code unit 0x0022 (QUOTATION MARK).
For each code unit
C in
value
If
C is 0x0022 (QUOTATION MARK) or 0x005C (REVERSE SOLIDUS), then
Let
product be the concatenation of
product and code unit 0x005C (REVERSE SOLIDUS).
Let
product be the concatenation of
product and
C.
Else if
C is 0x0008 (BACKSPACE), 0x000C (FORM FEED), 0x000A (LINE FEED), 0x000D (CARRIAGE RETURN), or 0x0009 (CHARACTER
TABULATION), then
Let
product be the concatenation of
product and code unit 0x005C (REVERSE SOLIDUS).
Let
abbrev be the String value corresponding to the value of
C as follows:
BACKSPACE
"b"
FORM FEED (FF)
"f"
LINE FEED (LF)
"n"
CARRIAGE RETURN (CR)
"r"
CHARACTER TABULATION
"t"
Let
product be the concatenation of
product and
abbrev.
Else if
C has a code unit value less than 0x0020 (SPACE), then
Let
product be the concatenation of
product and code unit 0x005C (REVERSE SOLIDUS).
Let
product be the concatenation of
product and
"u".
Let
hex be the string result of converting the numeric code unit value of
C to a String of four hexadecimal digits. Alphabetic hexadecimal digits are presented as lowercase Latin
letters.
Let
product be the concatenation of
product and
hex.
Else,
Let
product be the concatenation of
product and
C.
Let
product be the concatenation of
product and code unit 0x0022 (QUOTATION MARK).
Return
product.
24.3.2.3Runtime Semantics: SerializeJSONObject (
value )
#
The abstract operation SerializeJSONObject with argument
value serializes an object. It has access to the
stack,
indent,
gap, and
PropertyList values of the current invocation of the
stringify method.
If
stack contains
value, throw a
TypeError exception because the structure is cyclical.
Append
value to
stack.
Let
stepback be
indent.
Let
indent be the concatenation of
indent and
gap.
Let
member be the concatenation of
member and the string
":".
If
gap is not the empty String, then
Let
member be the concatenation of
member and code unit 0x0020 (SPACE).
Let
member be the concatenation of
member and
strP.
Append
member to
partial.
If
partial is empty, then
Let
final be
"{}".
Else,
If
gap is the empty String, then
Let
properties be a String formed by concatenating all the element Strings of
partial with each adjacent pair of Strings separated with code unit 0x002C (COMMA). A comma is not inserted
either before the first String or after the last String.
Let
final be the result of concatenating
"{",
properties, and
"}".
Else
gap is not the empty String
Let
separator be the result of concatenating code unit 0x002C (COMMA), code unit 0x000A (LINE FEED), and
indent.
Let
properties be a String formed by concatenating all the element Strings of
partial with each adjacent pair of Strings separated with
separator. The
separator String is not inserted either before the first String or after the last String.
Let
final be the result of concatenating
"{", code unit 0x000A (LINE FEED),
indent,
properties, code unit 0x000A (LINE FEED),
stepback, and
"}".
Remove the last element of
stack.
Let
indent be
stepback.
Return
final.
24.3.2.4Runtime Semantics: SerializeJSONArray (
value )
#
The abstract operation SerializeJSONArray with argument
value serializes an array. It has access to the
stack,
indent, and
gap values of the current invocation of the
stringify method.
If
stack contains
value, throw a
TypeError exception because the structure is cyclical.
Append
value to
stack.
Let
stepback be
indent.
Let
indent be the concatenation of
indent and
gap.
Let
properties be a String formed by concatenating all the element Strings of
partial with each adjacent pair of Strings separated with code unit 0x002C (COMMA). A comma is not inserted
either before the first String or after the last String.
Let
final be the result of concatenating
"[",
properties, and
"]".
Else,
Let
separator be the result of concatenating code unit 0x002C (COMMA), code unit 0x000A (LINE FEED), and
indent.
Let
properties be a String formed by concatenating all the element Strings of
partial with each adjacent pair of Strings separated with
separator. The
separator String is not inserted either before the first String or after the last String.
Let
final be the result of concatenating
"[", code unit 0x000A (LINE FEED),
indent,
properties, code unit 0x000A (LINE FEED),
stepback, and
"]".
Remove the last element of
stack.
Let
indent be
stepback.
Return
final.
Note
The representation of arrays includes only the elements between zero and
array.length - 1 inclusive. Properties whose keys are not array indexes are excluded from the stringification.
An array is stringified as an opening LEFT SQUARE BRACKET, elements separated by COMMA, and a closing RIGHT SQUARE
BRACKET.
An interface is a set of property keys whose associated values match a specific specification. Any object that provides
all the properties as described by an interface's specification
conforms to that interface. An interface is not represented by a distinct object. There may be many separately
implemented objects that conform to any interface. An individual object may conform to multiple interfaces.
An object that implements the
Iterator interface must include the property in
Table 54. Such objects may also implement the properties in
Table 55.
Table 54:
Iterator Interface Required Properties
Property
Value
Requirements
next
A function that returns an
IteratorResult object.
The returned object must conform to the
IteratorResult interface. If a previous call to the
next method of an
Iterator has returned an
IteratorResult object whose
done property is
true, then all subsequent calls to the
next method of that object should also return an
IteratorResult object whose
done property is
true. However, this requirement is not enforced.
Note 1
Arguments may be passed to the next function but their interpretation and validity is dependent upon the target
Iterator. The for-of statement and other common users of
Iterators do not pass any arguments, so
Iterator objects that expect to be used in such a manner must be prepared to deal with being called with no arguments.
Table 55:
Iterator Interface Optional Properties
Property
Value
Requirements
return
A function that returns an
IteratorResult object.
The returned object must conform to the
IteratorResult interface. Invoking this method notifies the
Iterator object that the caller does not intend to make any more
next method calls to the
Iterator. The returned
IteratorResult object will typically have a
done property whose value is
true, and a
value property with the value passed as the argument of the
return method. However, this requirement is not enforced.
throw
A function that returns an
IteratorResult object.
The returned object must conform to the
IteratorResult interface. Invoking this method notifies the
Iterator object that the caller has detected an error condition. The argument may be used to identify the
error condition and typically will be an exception object. A typical response is to
throw the value passed as the argument. If the method does not
throw, the returned
IteratorResult object will typically have a
done property whose value is
true.
Note 2
Typically callers of these methods should check for their existence before invoking them. Certain ECMAScript language
features including
for-
of,
yield*, and array destructuring call these methods after performing an existence check. Most ECMAScript library
functions that accept
Iterable objects as arguments also conditionally call them.
The
IteratorResult interface includes the properties listed in
Table 56:
Table 56:
IteratorResult Interface Properties
Property
Value
Requirements
done
Either
true or
false.
This is the result status of an
iteratornext method call. If the end of the iterator was reached
done is
true. If the end was not reached
done is
false and a value is available. If a
done property (either own or inherited) does not exist, it is consider to have the value
false.
If done is
false, this is the current iteration element value. If done is
true, this is the return value of the iterator, if it supplied one. If the iterator does not have a
return value,
value is
undefined. In that case, the
value property may be absent from the conforming object if it does not inherit an explicit
value property.
The value of the [[Prototype]] internal slot of the
%IteratorPrototype% object is the intrinsic object
%ObjectPrototype%. The %IteratorPrototype% object is an ordinary object. The initial value of the [[Extensible]] internal slot of the
%IteratorPrototype% object is
true.
Note
All objects defined in this specification that implement the Iterator interface also inherit from %IteratorPrototype%.
ECMAScript code may also define objects that inherit from %IteratorPrototype%.The %IteratorPrototype% object provides
a place where additional methods that are applicable to all iterator objects may be added.
The following expression is one way that ECMAScript code can access the %IteratorPrototype% object:
The
GeneratorFunction constructor is the
%GeneratorFunction% intrinsic. When
GeneratorFunction is called as a function rather than as a constructor, it creates and initializes a new GeneratorFunction
object. Thus the function call
GeneratorFunction (…) is equivalent to the object creation expression
new GeneratorFunction (…) with the same arguments.
GeneratorFunction is designed to be subclassable. It may be used as the value of an
extends clause of a class definition. Subclass constructors that intend to inherit the specified
GeneratorFunction behaviour must include a
super call to the
GeneratorFunction constructor to create and initialize subclass instances with the internal slots necessary for
built-in GeneratorFunction behaviour. All ECMAScript syntactic forms for defining generator function objects create
direct instances of
GeneratorFunction. There is no syntactic means to create instances of
GeneratorFunction subclasses.
The last argument specifies the body (executable code) of a generator function; any preceding arguments specify formal
parameters.
When the
GeneratorFunction function is called with some arguments
p1,
p2, … ,
pn,
body (where
n might be 0, that is, there are no “
p” arguments, and where
body might also not be provided), the following steps are taken:
25.2.2Properties of the GeneratorFunction Constructor
#
The
GeneratorFunction constructor is a standard built-in function object that inherits from the
Function constructor. The value of the [[Prototype]] internal slot of the
GeneratorFunction constructor is the intrinsic object
%Function%.
The value of the [[Extensible]] internal slot of the GeneratorFunction constructor is
true.
The value of the
name property of the GeneratorFunction is
"GeneratorFunction".
The
GeneratorFunction constructor has the following properties:
The initial value of
GeneratorFunction.prototype is the intrinsic object %Generator%.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
false }.
25.2.3Properties of the GeneratorFunction Prototype Object
#
The GeneratorFunction prototype object is an ordinary object. It is not a function object and does not have an [[ECMAScriptCode]]
internal slot or any other of the internal slots listed in
Table 27 or
Table 57. In addition to being the value of the prototype property of the
%GeneratorFunction% intrinsic, it is the %Generator% intrinsic (see Figure 2).
The value of the [[Prototype]] internal slot of the GeneratorFunction prototype object is the
%FunctionPrototype% intrinsic object. The initial value of the [[Extensible]] internal slot of the GeneratorFunction prototype object is
true.
Every GeneratorFunction instance is an ECMAScript function object and has the internal slots listed in
Table 27. The value of the [[FunctionKind]] internal slot for all such instances is
"generator".
Each GeneratorFunction instance has the following own properties:
The value of the
length property is an integer that indicates the typical number of arguments expected by the GeneratorFunction.
However, the language permits the function to be invoked with some other number of arguments. The behaviour of a GeneratorFunction
when invoked on a number of arguments other than the number specified by its
length property depends on the function.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
Whenever a GeneratorFunction instance is created another ordinary object is also created and is the initial value of
the generator function's
prototype property. The value of the prototype property is used to initialize the [[Prototype]] internal slot
of a newly created Generator object when the generator function object is invoked using [[Call]].
This property has the attributes { [[Writable]]:
true, [[Enumerable]]:
false, [[Configurable]]:
false }.
Note
Unlike function instances, the object that is the value of the a GeneratorFunction's
prototype property does not have a
constructor property whose value is the GeneratorFunction instance.
A Generator object is an instance of a generator function and conforms to both the
Iterator and
Iterable interfaces.
Generator instances directly inherit properties from the object that is the value of the
prototype property of the Generator function that created the instance. Generator instances indirectly inherit
properties from the Generator Prototype intrinsic,
%GeneratorPrototype%.
The Generator prototype object is the
%GeneratorPrototype% intrinsic. It is also the initial value of the
prototype property of the %Generator% intrinsic (the GeneratorFunction.prototype).
The Generator prototype is an ordinary object. It is not a Generator instance and does not have a [[GeneratorState]]
internal slot.
The value of the [[Prototype]] internal slot of the Generator prototype object is the intrinsic object
%IteratorPrototype%. The initial value of the [[Extensible]] internal slot of the Generator prototype object is
true.
All Generator instances indirectly inherit properties of the Generator prototype object.
Set
generator's [[GeneratorState]] internal slot to
"completed".
Once a generator enters the
"completed" state it never leaves it and its associated
execution context is never resumed. Any execution state associated with
generator can be discarded at this point.
If
result is a normal completion, let
resultValue be
undefined.
Else,
If
result.[[Type]] is
return, let
resultValue be
result.[[Value]].
Resume the suspended evaluation of
genContext using
NormalCompletion(
value) as the result of the operation that suspended it. Let
result be the value returned by the resumed computation.
Set
generator's [[GeneratorState]] internal slot to
"completed".
Once a generator enters the
"completed" state it never leaves it and its associated
execution context is never resumed. Any execution state associated with
generator can be discarded at this point.
Resume the suspended evaluation of
genContext using
abruptCompletion as the result of the operation that suspended it. Let
result be the completion record returned by the resumed computation.
Set the code evaluation state of
genContext such that when evaluation is resumed with a
CompletionresumptionValue the following steps will be performed:
Return
resumptionValue.
NOTE: This returns to the evaluation of the
YieldExpression production that originally called this abstract operation.
任何Promise对象都处于三种互斥状态之一：
fulfilled,
rejected, and
pending:
一个promise
p 是
fulfilled并且
p.then(f, r)，将立即将Job排入队列以调用函数
f.
一个promise
p 是
rejected并且
p.then(f, r)，将立即将Job排入队列以调用函数
r.
一个promise
p 既不是
fulfilled也不是
rejected，则为
pending .
A promise is said to be
settled if it is not pending, i.e. if it is either fulfilled or rejected.
A promise is
resolved if it is settled or if it has been “locked in” to match the state of another promise. Attempting to resolve
or reject a resolved promise has no effect. A promise is
unresolved if it is not resolved. An unresolved promise is always in the pending state. A resolved promise may be
pending, fulfilled or rejected.
If
IsCallable(
promiseCapability.[[Resolve]]) is
false, throw a
TypeError exception.
If
IsCallable(
promiseCapability.[[Reject]]) is
false, throw a
TypeError exception.
Set
promiseCapability.[[Promise]] to
promise.
Return
promiseCapability.
Note
This abstract operation supports Promise subclassing, as it is generic on any constructor that calls a passed executor
function argument in the same way as the Promise constructor. It is used to generalize static methods of the Promise
constructor to any subclass.
A typical implementation of HostPromiseRejectionTracker might try to notify developers of unhandled rejections, while
also being careful to notify them if such previous notifications are later invalidated by new handlers being attached.
Note 2
If
operation is
"handle", 则实现(implementation)不应以对干扰垃圾收集的方式持有对promise的引用.
if operation is
"reject" 则实现可以保留对promise的引用, since it is expected that rejections will be rare and not on hot code paths.
This Job uses the supplied thenable and its
then method to resolve the given promise. This process must take place as a Job to ensure that the evaluation
of the
then method occurs after evaluation of any surrounding code has completed.
The
all function returns a new promise which is fulfilled with an array of fulfillment values for the passed promises,
or rejects with the reason of the first passed promise that rejects. It resolves all elements of the passed iterable
to promises as it runs this algorithm.
Let
C be the
this value.
If
Type(
C) is not Object, throw a
TypeError exception.
When the PerformPromiseAll abstract operation is called with arguments
iteratorRecord,
constructor, and
resultCapability, the following steps are taken:
Assert:
constructor is a constructor function.
Assert:
resultCapability is a PromiseCapability record.
A Promise.all resolve element function is an anonymous built-in function that is used to resolve a specific Promise.all
element. Each Promise.all resolve element function has [[Index]], [[Values]], [[Capabilities]], [[RemainingElements]],
and [[AlreadyCalled]] internal slots.
When a Promise.all resolve element function
F is called with argument
x, the following steps are taken:
Let
alreadyCalled be the value of
F's [[AlreadyCalled]] internal slot.
If
alreadyCalled.[[Value]] is
true, return
undefined.
Set
alreadyCalled.[[Value]] to
true.
Let
index be the value of
F's [[Index]] internal slot.
Let
values be the value of
F's [[Values]] internal slot.
Let
promiseCapability be the value of
F's [[Capabilities]] internal slot.
Let
remainingElementsCount be the value of
F's [[RemainingElements]] internal slot.
Set
values[
index] to
x.
Set
remainingElementsCount.[[Value]] to
remainingElementsCount.[[Value]] - 1.
The
race function returns a new promise which is settled in the same way as the first passed promise to settle.
It resolves all elements of the passed
iterable to promises as it runs this algorithm.
Let
C be the
this value.
If
Type(
C) is not Object, throw a
TypeError exception.
If the
iterable argument is empty or if none of the promises in
iterable ever settle then the pending promise returned by this method will never be settled.
Note 2
The
race function expects its
this value to be a constructor function that supports the parameter conventions of the
Promise constructor. It also expects that its
this value provides a
resolve method.
25.4.4.3.1Runtime Semantics: PerformPromiseRace (
iteratorRecord,
promiseCapability,
C )
#
When the PerformPromiseRace abstract operation is called with arguments
iteratorRecord,
promiseCapability, and
C, the following steps are taken:
Repeat
Let
next be
IteratorStep(
iteratorRecord.[[Iterator]]).
If
next is an
abrupt completion, set
iteratorRecord.[[Done]] to
true.
The
resolve function returns either a new promise resolved with the passed argument, or the argument itself if the
argument is a promise produced by this constructor.
Let
C be the
this value.
If
Type(
C) is not Object, throw a
TypeError exception.
Promise[@@species] is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps:
Return the
this value.
The value of the
name property of this function is
"get [Symbol.species]".
Note
Promise prototype methods normally use their
this object's constructor to create a derived object. However, a subclass constructor may over-ride that default
behaviour by redefining its @@species property.
25.4.5Properties of the Promise Prototype Object
#
The abstract operation PerformPromiseThen performs the “then” operation on
promise using
onFulfilled and
onRejected as its settlement actions. The result is
resultCapability's promise.
Promise instances are ordinary objects that inherit properties from the Promise prototype object (the intrinsic,
%PromisePrototype%). Promise instances are initially created with the internal slots described in
Table 60.
Table 60: Internal Slots of Promise Instances
Internal Slot
Description
[[PromiseState]]
A String value that governs how a promise will react to incoming calls to its
then method. The possible values are:
"pending",
"fulfilled", and
"rejected".
[[PromiseResult]]
The value with which the promise has been fulfilled or rejected, if any. Only meaningful if [[PromiseState]] is not
"pending".
[[PromiseFulfillReactions]]
A
List of PromiseReaction records to be processed when/if the promise transitions from the
"pending" state to the
"fulfilled" state.
[[PromiseRejectReactions]]
A
List of PromiseReaction records to be processed when/if the promise transitions from the
"pending" state to the
"rejected" state.
[[PromiseIsHandled]]
A boolean indicating whether the promise has ever had a fulfillment or rejection handler; used in unhandled rejection tracking.
The Reflect object is the
%Reflect% intrinsic object and the initial value of the
Reflect property of the
global object. The Reflect object is an ordinary object.
The value of the [[Prototype]] internal slot of the Reflect object is the intrinsic object
%ObjectPrototype%.
The Reflect object is not a function object. It does not have a [[Construct]] internal method; it is not possible to use
the Reflect object as a constructor with the
new operator. The Reflect object also does not have a [[Call]] internal method; it is not possible to invoke the
Reflect object as a function.
The Proxy constructor is the
%Proxy% intrinsic object and the initial value of the
Proxy property of the
global object. When called as a constructor it creates and initializes a new proxy exotic object.
Proxy is not intended to be called as a function and will throw an exception when called in that manner.
The value of the [[Prototype]] internal slot of the
Proxy constructor is the intrinsic object
%FunctionPrototype%.
The
Proxy constructor does not have a
prototype property because proxy exotic objects do not have a [[Prototype]] internal slot that requires initialization.
The
Proxy constructor has the following properties:
The
Proxy.revocable function is used to create a revocable Proxy object. When
Proxy.revocable is called with arguments
target and
handler, the following steps are taken:
A Module Namespace Object is a module namespace exotic object that provides runtime property-based access to a module's
exported bindings. There is no constructor function for Module Namespace Objects. Instead, such an object is created
for each module that is imported by an
ImportDeclaration that includes a
NameSpaceImport.
In addition to the properties specified in
9.4.6 each Module Namespace Object has the following own properties:
BAdditional ECMAScript Features for Web Browsers
#
The ECMAScript language syntax and semantics defined in this annex are required when the ECMAScript host is a web browser.
The content of this annex is normative but optional if the ECMAScript host is not a web browser.
Note
This annex describes various legacy features and other characteristics of web browser based ECMAScript implementations.
All of the language features and behaviours specified in this annex have one or more undesirable characteristics and
in the absence of legacy usage would be removed from this specification. However, the usage of these features by large
numbers of existing web pages means that web browsers must continue to support them. The specifications in this annex
define the requirements for interoperable implementations of these legacy features.
These features are not considered part of the core ECMAScript language. Programmers should not use or assume the existence
of these features and behaviours when writing new ECMAScript code. ECMAScript implementations are discouraged from implementing
these features unless the implementation is part of a web browser or is required to run the same legacy ECMAScript code
that web browsers encounter.
The syntax and semantics of
11.4 is extended as follows except that this extension is not allowed when parsing source code using the goal symbol
Module:
The syntax of
21.2.1 is modified and extended as follows. These changes introduce ambiguities that are broken by the ordering of grammar
productions and by contextual information. When parsing using the following grammar, each alternative is considered
only if previous production alternatives do not match.
This alternative pattern grammar and semantics only changes the syntax and semantics of BMP patterns. The following grammar
extensions include productions parameterized with the [U] parameter. However, none of these extensions change the syntax
of Unicode patterns recognized when parsing with the [U] parameter present on the goal symbol.
The
escape function is a property of the
global object. It computes a new version of a String value in which certain code units have been replaced by a hexadecimal escape
sequence.
For those code units being replaced whose value is
0x00FF or less, a two-digit escape sequence of the form
%
xx is used. For those characters being replaced whose code unit value is greater than
0x00FF, a four-digit escape sequence of the form
%u
xxxx is used.
The
escape function is the %escape% intrinsic object. When the
escape function is called with one argument
string, the following steps are taken:
Let
char be the code unit (represented as a 16-bit unsigned integer) at index
k within
string.
If
char is one of the code units in
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789@*_+-./", then
Let
S be a String containing the single code unit
char.
Else if
char ≥ 256, then
Let
S be a String containing six code units
"%u
wxyz" where
wxyz are the code units of the four hexadecimal digits encoding the value of
char.
Else,
char < 256
Let
S be a String containing three code units
"%
xy" where
xy are the code units of two hexadecimal digits encoding the value of
char.
Let
R be a new String value computed by concatenating the previous value of
R and
S.
Increase
k by 1.
Return
R.
Note
The encoding is partly based on the encoding described in RFC 1738, but the entire encoding specified in this standard
is described above without regard to the contents of RFC 1738. This encoding does not reflect changes to RFC 1738
made by RFC 3986.
The
unescape function is a property of the
global object. It computes a new version of a String value in which each escape sequence of the sort that might be introduced by
the
escape function is replaced with the code unit that it represents.
The
unescape function is the %unescape% intrinsic object. When the
unescape function is called with one argument
string, the following steps are taken:
If
k ≤
length-6 and the code unit at index
k+1 within
string is
u and the four code units at indices
k+2,
k+3,
k+4, and
k+5 within
string are all hexadecimal digits, then
Let
c be the code unit whose value is the integer represented by the four hexadecimal digits at indices
k+2,
k+3,
k+4, and
k+5 within
string.
Increase
k by 5.
Else if
k ≤
length-3 and the two code units at indices
k+1 and
k+2 within
string are both hexadecimal digits, then
Let
c be the code unit whose value is the integer represented by two zeroes plus the two hexadecimal digits
at indices
k+1 and
k+2 within
string.
Increase
k by 2.
Let
R be a new String value computed by concatenating the previous value of
R and
c.
Increase
k by 1.
Return
R.
B.2.2Additional Properties of the Object.prototype Object
#
Object.prototype.__proto__ is an accessor property with attributes { [[Enumerable]]:
false, [[Configurable]]:
true }. The [[Get]] and [[Set]] attributes are defined as follows:
The
substr method takes two arguments,
start and
length, and returns a substring of the result of converting the
this object to a String, starting from index
start and running for
length code units (or through the end of the String if
length is
undefined). If
start is negative, it is treated as
sourceLength+
start where
sourceLength is the length of the String. The result is a String value, not a String object. The following steps
are taken:
If
length is
undefined, let
end be
+∞; otherwise let
end be ?
ToInteger(
length).
Let
size be the number of code units in
S.
If
intStart < 0, let
intStart be
max(
size +
intStart, 0).
Let
resultLength be
min(
max(
end, 0),
size -
intStart).
If
resultLength ≤ 0, return the empty String
"".
Return a String containing
resultLength consecutive code units from
S beginning with the code unit at index
intStart.
Note
The
substr function is intentionally generic; it does not require that its
this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
B.2.3.2.1Runtime Semantics: CreateHTML (
string,
tag,
attribute,
value )
#
The abstract operation
CreateHTML is called with arguments
string,
tag,
attribute, and
value. The arguments
tag and
attribute must be String values. The following steps are taken:
Let
escapedV be the String value that is the same as
V except that each occurrence of the code unit 0x0022 (QUOTATION MARK) in
V has been replaced with the six code unit sequence
""".
Let
p1 be the String value that is the concatenation of the following String values:
The String value of
p1
Code unit 0x0020 (SPACE)
The String value of
attribute
Code unit 0x003D (EQUALS SIGN)
Code unit 0x0022 (QUOTATION MARK)
The String value of
escapedV
Code unit 0x0022 (QUOTATION MARK)
Let
p2 be the String value that is the concatenation of
p1 and
">".
Let
p3 be the String value that is the concatenation of
p2 and
S.
Let
p4 be the String value that is the concatenation of
p3,
"</",
tag, and
">".
The property
toUTCString is preferred. The
toGMTString property is provided principally for compatibility with old code. It is recommended that the
toUTCString property be used in new ECMAScript code.
The function object that is the initial value of
Date.prototype.toGMTString is the same function object that is the initial value of
Date.prototype.toUTCString.
B.2.5Additional Properties of the RegExp.prototype Object
#
The
compile method completely reinitializes the
this object RegExp with a new pattern and flags. An implementation may interpret use of this method as an
assertion that the resulting RegExp object will be used multiple times and hence is a candidate for extra optimization.
Prior to ECMAScript 2015, the specification of
LabelledStatement did not allow for the association of a statement label with a
FunctionDeclaration. However, a labelled
FunctionDeclaration was an allowable extension for non-strict code and most browser-hosted ECMAScript implementations supported that extension.
In ECMAScript 2015, the grammar productions for
LabelledStatement permits use of
FunctionDeclaration as a
LabelledItem but
13.13.1 includes an Early Error rule that produces a Syntax Error if that occurs. For web browser compatibility, that rule is
modified with the addition of the
highlighted text:
It is a Syntax Error if any
strict mode source code matches this rule.
B.3.3Block-Level Function Declarations Web Legacy Compatibility Semantics
#
Prior to ECMAScript 2015, the ECMAScript specification did not define the occurrence of a
FunctionDeclaration as an element of a
Block statement's
StatementList. However, support for that form of
FunctionDeclaration was an allowable extension and most browser-hosted ECMAScript implementations permitted them. Unfortunately, the semantics
of such declarations differ among those implementations. Because of these semantic differences, existing web ECMAScript
code that uses
Block level function declarations is only portable among browser implementation if the usage only depends upon the semantic
intersection of all of the browser implementations for such declarations. The following are the use cases that fall
within that intersection semantics:
A function is declared and only referenced within a single block
A
FunctionDeclaration whose
BindingIdentifier is the name
f occurs exactly once within the function code of an enclosing function
g and that declaration is nested within a
Block.
No other declaration of
f that is not a
var declaration occurs within the function code of
g
A function is declared and possibly used within a single
Block but also referenced by an inner function definition that is not contained within that same
Block.
A
FunctionDeclaration whose
BindingIdentifier is the name
f occurs exactly once within the function code of an enclosing function
g and that declaration is nested within a
Block.
No other declaration of
f that is not a
var declaration occurs within the function code of
g
There is at least one occurrence of
f as an
IdentifierReference within another function
h that is nested within
g and no other declaration of
f shadows the references to
f from within
h.
All invocations of
h occur after the declaration of
f has been evaluated.
A function is declared and possibly used within a single block but also referenced within subsequent blocks.
A
FunctionDeclaration whose
BindingIdentifier is the name
f occurs exactly once within the function code of an enclosing function
g and that declaration is nested within a
Block.
No other declaration of
f that is not a
var declaration occurs within the function code of
g
There is at least one occurrence of
f as an
IdentifierReference within the function code of
g that lexically follows the
Block containing the declaration of
f.
The first use case is interoperable with the semantics of
Block level function declarations provided by ECMAScript 2015. Any pre-existing ECMAScript code that employs that use case
will operate using the Block level function declarations semantics defined by clauses 9, 13, and 14 of this specification.
ECMAScript 2015 interoperability for the second and third use cases requires the following extensions to the clause
9, clause
14, clause
18.2.1 and clause
15.1.11 semantics.
If an ECMAScript implementation has a mechanism for reporting diagnostic warning messages, a warning should be produced
when code contains a
FunctionDeclaration for which these compatibility semantics are applied and introduce observable differences from non-compatibility semantics.
For example, if a var binding is not introduced because its introduction would create an
early error, a warning message should not be produced.
B.3.3.1Changes to FunctionDeclarationInstantiation
#
The above rules are only applied when parsing code that is not
strict mode code. If any such code is match by one of these rules subsequent processing of that code takes places as if each matching
occurrence of
FunctionDeclaration[?Yield] was the sole
StatementListItem of a
BlockStatement occupying that position in the source code. The semantics of such a synthetic
BlockStatement includes the web legacy compatibility semantics specified in
B.3.3.
The
Block of a
Catch clause may contain
var declarations that bind a name that is also bound by the
CatchParameter. At runtime, such bindings are instantiated in the VariableDeclarationEnvironment. They do not shadow the same-named
bindings introduced by the
CatchParameter and hence the
Initializer for such
var declarations will assign to the corresponding catch parameter rather than the
var binding. The relaxation of the normal static semantic rule does not apply to names only bound by for-of
statements.
This modified behaviour also applies to
var and
function declarations introduced by
direct eval calls contained within the
Block of a
Catch clause. This change is accomplished by modify the algorithm of
18.2.1.2 as follows:
Step 5.d.ii.2.a.i is replaced by:
If
thisEnvRec is not the
Environment Record for a
Catch clause, throw a
SyntaxError exception.
Assignment to an undeclared identifier or otherwise unresolvable reference does not create a property in the
global object. When a simple assignment occurs within
strict mode code, its
LeftHandSideExpression must not evaluate to an unresolvable
Reference. If it does a
ReferenceError exception is thrown (
6.2.3.2). The
LeftHandSideExpression also may not be a reference to a data property with the attribute value {[[Writable]]:
false}, to an accessor property with the attribute value {[[Set]]:
undefined}, nor to a non-existent property of an object whose [[Extensible]] internal slot has the value
false. In these cases a
TypeError exception is thrown (
12.15).
Arguments objects for strict mode functions define non-configurable accessor properties named
"caller" and
"callee" which throw a
TypeError exception on access (
9.2.7).
Arguments objects for strict mode functions do not dynamically share their array indexed property values with the corresponding
formal parameter bindings of their functions. (
9.4.4).
For strict mode functions, if an arguments object is created the binding of the local identifier
arguments to the arguments object is immutable and hence may not be the target of an assignment expression. (
9.2.12).
Strict mode eval code cannot instantiate variables or functions in the variable environment of the caller to eval. Instead,
a new variable environment is created and that environment is used for declaration binding instantiation for the eval
code (
18.2.1).
If
this is evaluated within
strict mode code, then the
this value is not coerced to an object. A
this value of
null or
undefined is not converted to the
global object and primitive values are not converted to wrapper objects. The
this value passed via a function call (including calls made using
Function.prototype.apply and
Function.prototype.call) do not coerce the passed this value to an object (
9.2.1.2,
19.2.3.1,
19.2.3.3).
When a
delete operator occurs within
strict mode code, a
SyntaxError is thrown if its
UnaryExpression is a direct reference to a variable, function argument, or function name (
12.5.3.1).
When a
delete operator occurs within
strict mode code, a
TypeError is thrown if the property to be deleted has the attribute { [[Configurable]]:
false } (
12.5.3.2).
It is a
SyntaxError if the same
BindingIdentifier appears more than once in the
FormalParameters of a strict mode function. An attempt to create such a function using a
Function or
Generator constructor is a
SyntaxError (
14.1.2,
19.2.1.1.1).
An implementation may not extend, beyond that defined in this specification, the meanings within strict mode functions of
properties named
caller or
arguments of function instances. ECMAScript code may not create or modify properties with these names on function
objects that correspond to strict mode functions (
16.2).
DCorrections and Clarifications in ECMAScript 2015 with Possible Compatibility Impact
#
8.1.1.4.15-
8.1.1.4.18 Edition 5 and 5.1 used a property existence test to determine whether a
global object property corresponding to a new global declaration already existed. ECMAScript 2015 uses an own property existence test.
This corresponds to what has been most commonly implemented by web browsers.
9.4.2.1: The 5
th Edition moved the capture of the current array length prior to the integer conversion of the array index or new
length value. However, the captured length value could become invalid if the conversion process has the side-effect of
changing the array length. ECMAScript 2015 specifies that the current array length must be captured after the possible
occurrence of such side-effects.
20.3.1.15: Previous editions permitted the
TimeClip abstract operation to return either
+0 or
-0 as the representation of a 0
time value. ECMAScript 2015 specifies that
+0 always returned. This means that for ECMAScript 2015 the
time value of a Date object is never observably
-0 and methods that return time values never return
-0.
20.3.1.16: If a time zone offset is not present, the local time zone is used. Edition 5.1 incorrectly stated that a missing time
zone should be interpreted as
"z".
20.3.4.36: If the year cannot be represented using the Date Time String Format specified in
20.3.1.16 a RangeError exception is thrown. Previous editions did not specify the behaviour for that case.
20.3.4.41: Previous editions did not specify the value returned by Date.prototype.toString when
this time value is
NaN. ECMAScript 2015 specifies the result to be the String value is
"Invalid Date".
21.2.3.1,
21.2.3.2.4: Any LineTerminator code points in the value of the
source property of an RegExp instance must be expressed using an escape sequence. Edition 5.1 only required the
escaping of
"/".
21.2.5.6,
21.2.5.8: In previous editions, the specifications for
String.prototype.match and
String.prototype.replace was incorrect for cases where the pattern argument was a RegExp value whose
global is flag set. The previous specifications stated that for each attempt to match the pattern, if
lastIndex did not change it should be incremented by 1. The correct behaviour is that
lastIndex should be incremented by one only if the pattern matched the empty string.
22.1.3.25,
22.1.3.25.1: Previous editions did not specify how a
NaN value returned by a
comparefn was interpreted by
Array.prototype.sort. ECMAScript 2015 specifies that such as value is treated as if
+0 was returned from the
comparefn. ECMAScript 2015 also specifies that
ToNumber is applied to the result returned by a
comparefn. In previous editions, the effect of a
comparefn result that is not a Number value was implementation dependent. In practice, implementations call
ToNumber.
EAdditions and Changes That Introduce Incompatibilities with Prior Editions
#
6.2.3: In ECMAScript 2015, Function calls are not allowed to return a
Reference value.
11.6: In ECMAScript 2015, the valid code points for an
IdentifierName are specified in terms of the Unicode properties “ID_Start” and “ID_Continue”. In previous editions, the valid
IdentifierName or
Identifier code points were specified by enumerating various Unicode code point categories.
11.9.1: In ECMAScript 2015, Automatic Semicolon Insertion adds a semicolon at the end of a do-while statement if the semicolon
is missing. This change aligns the specification with the actual behaviour of most existing implementations.
12.2.6.1: In ECMAScript 2015, it is no longer an
early error to have duplicate property names in Object Initializers.
12.15.1: In ECMAScript 2015,
strict mode code containing an assignment to an immutable binding such as the function name of a
FunctionExpression does not produce an
early error. Instead it produces a runtime error.
13.2: In ECMAScript 2015, a
StatementList beginning with the token let followed by the input elements
LineTerminator then
Identifier is the start of a
LexicalDeclaration. In previous editions, automatic semicolon insertion would always insert a semicolon before the
Identifier input element.
13.6.7: In ECMAScript 2015, the normal completion value of an
IfStatement is never the value
empty. If no
Statement part is evaluated or if the evaluated
Statement part produces a normal completion whose value is
empty, the completion value of the
IfStatement is
undefined.
13.7: In ECMAScript 2015, if the
( token of a for statement is immediately followed by the token sequence
let [ then the
let is treated as the start of a
LexicalDeclaration. In previous editions such a token sequence would be the start of an
Expression.
13.7: In ECMAScript 2015, if the ( token of a for-in statement is immediately followed by the token sequence
let [ then the
let is treated as the start of a
ForDeclaration. In previous editions such a token sequence would be the start of an
LeftHandSideExpression.
13.7: Prior to ECMAScript 2015, an initialization expression could appear as part of the
VariableDeclaration that precedes the
in keyword. The value of that expression was always discarded. In ECMAScript 2015, the
ForBinding in that same position does not allow the occurrence of such an initializer.
13.7: In ECMAScript 2015, the completion value of an
IterationStatement is never the value
empty. If the
Statement part of an
IterationStatement is not evaluated or if the final evaluation of the
Statement part produces a completion whose value is
empty, the completion value of the
IterationStatement is
undefined.
13.11.7: In ECMAScript 2015, the normal completion value of a
WithStatement is never the value
empty. If evaluation of the
Statement part of a
WithStatement produces a normal completion whose value is
empty, the completion value of the
WithStatement is
undefined.
13.15: In ECMAScript 2015, it is an
early error for a
Catch clause to contain a
var declaration for the same
Identifier that appears as the
Catch clause parameter. In previous editions, such a variable declaration would be instantiated in the enclosing variable environment
but the declaration's
Initializer value would be assigned to the
Catch parameter.
13.15,
18.2.1.2: In ECMAScript 2015, a runtime
SyntaxError is thrown if a
Catch clause evaluates a non-strict direct
eval whose eval code includes a
var or
FunctionDeclaration declaration that binds the same
Identifier that appears as the
Catch clause parameter.
13.15.8: In ECMAScript 2015, the completion value of a
TryStatement is never the value
empty. If the
Block part of a
TryStatement evaluates to a normal completion whose value is
empty, the completion value of the
TryStatement is
undefined. If the
Block part of a
TryStatement evaluates to a throw completion and it has a
Catch part that evaluates to a normal completion whose value is
empty, the completion value of the
TryStatement is
undefined if there is no
Finally clause or if its
Finally clause evalulates to an
empty normal completion.
14.3.9 In ECMAScript 2015, the function objects that are created as the values of the [[Get]] or [[Set]] attribute of accessor
properties in an
ObjectLiteral are not constructor functions and they do not have a
prototype own property. In the previous edition, they were constructors and had a
prototype property.
19.1.2.5: In ECMAScript 2015, if the argument to
Object.freeze is not an object it is treated as if it was a non-extensible ordinary object with no own properties.
In the previous edition, a non-object argument always causes a
TypeError to be thrown.
19.1.2.6: In ECMAScript 2015, if the argument to
Object.getOwnPropertyDescriptor is not an object an attempt is made to coerce the argument using
ToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a
non-object argument always causes a
TypeError to be thrown.
19.1.2.7: In ECMAScript 2015, if the argument to
Object.getOwnPropertyNames is not an object an attempt is made to coerce the argument using
ToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a
non-object argument always causes a
TypeError to be thrown.
19.1.2.9: In ECMAScript 2015, if the argument to
Object.getPrototypeOf is not an object an attempt is made to coerce the argument using
ToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a
non-object argument always causes a
TypeError to be thrown.
19.1.2.11: In ECMAScript 2015, if the argument to
Object.isExtensible is not an object it is treated as if it was a non-extensible ordinary object with no own properties.
In the previous edition, a non-object argument always causes a
TypeError to be thrown.
19.1.2.12: In ECMAScript 2015, if the argument to
Object.isFrozen is not an object it is treated as if it was a non-extensible ordinary object with no own properties.
In the previous edition, a non-object argument always causes a
TypeError to be thrown.
19.1.2.13: In ECMAScript 2015, if the argument to
Object.isSealed is not an object it is treated as if it was a non-extensible ordinary object with no own properties.
In the previous edition, a non-object argument always causes a
TypeError to be thrown.
19.1.2.14: In ECMAScript 2015, if the argument to
Object.keys is not an object an attempt is made to coerce the argument using
ToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a
non-object argument always causes a
TypeError to be thrown.
19.1.2.15: In ECMAScript 2015, if the argument to
Object.preventExtensions is not an object it is treated as if it was a non-extensible ordinary object with no own
properties. In the previous edition, a non-object argument always causes a
TypeError to be thrown.
19.1.2.17: In ECMAScript 2015, if the argument to
Object.seal is not an object it is treated as if it was a non-extensible ordinary object with no own properties.
In the previous edition, a non-object argument always causes a
TypeError to be thrown.
19.2.3.2: In ECMAScript 2015, the [[Prototype]] internal slot of a
bound function is set to the [[GetPrototypeOf]] value of its target function. In the previous edition, [[Prototype]] was always set to
%FunctionPrototype%.
19.2.4.1: In ECMAScript 2015, the
length property of function instances is configurable. In previous editions it was non-configurable.
19.5.6.2: In ECMAScript 2015, the [[Prototype]] internal slot of a
NativeError constructor is the Error constructor. In previous editions it was the Function prototype object.
20.3.4 In ECMAScript 2015, the Date prototype object is not a Date instance. In previous editions it was a Date instance whose
TimeValue was
NaN.
21.1.3.10 In ECMAScript 2015, the
String.prototype.localeCompare function must treat Strings that are canonically equivalent according to the Unicode
standard as being identical. In previous editions implementations were permitted to ignore canonical equivalence and could
instead use a bit-wise comparison.
21.1.3.22 and
21.1.3.24 In ECMAScript 2015, lowercase/upper conversion processing operates on code points. In previous editions such the conversion
processing was only applied to individual code units. The only affected code points are those in the Deseret block of
Unicode
21.1.3.25 In ECMAScript 2015, the
String.prototype.trim method is defined to recognize white space code points that may exists outside of the Unicode
BMP. However, as of Unicode 7 no such code points are defined. In previous editions such code points would not have been
recognized as white space.
21.2.3.1 In ECMAScript 2015, If the
pattern argument is a RegExp instance and the
flags argument is not
undefined, a new RegExp instance is created just like
pattern except that
pattern's flags are replaced by the argument
flags. In previous editions a
TypeError exception was thrown when
pattern was a RegExp instance and
flags was not
undefined.
21.2.5 In ECMAScript 2015, the RegExp prototype object is not a RegExp instance. In previous editions it was a RegExp instance
whose pattern is the empty string.
21.2.5 In ECMAScript 2015,
source,
global,
ignoreCase, and
multiline are accessor properties defined on the RegExp prototype object. In previous editions they were data properties
defined on RegExp instances
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